Method and apparatus for adjusting control parameters for cardiac event sensing

ABSTRACT

A medical device includes a motion sensor configured to produce a motion signal and a control circuit configured to set sensing control parameters and sense atrial events from the motion signal during ventricular cycles according to the sensing control parameters. In some examples, the control circuit is configured to determine a feature of the motion signal for at least some ventricular cycles, determine a metric of the motion signal based on the determined features, and adjust at least one of the sensing control parameters based on the metric.

REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Patent Application No.62/967,917, filed provisionally on Jan. 30, 2020, incorporated herein byreference in its entirety.

TECHNICAL FIELD

This disclosure relates to a medical device and method for adjustingcontrol parameters for sensing cardiac events from a motion sensorsignal.

BACKGROUND

Implantable cardiac pacemakers are often placed in a subcutaneous pocketand coupled to one or more transvenous medical electrical leads carryingpacing and sensing electrodes positioned in the heart. A cardiacpacemaker implanted subcutaneously may be a single chamber pacemakercoupled to one transvenous medical lead for positioning electrodes inone heart chamber, atrial or ventricular, or a dual chamber pacemakercoupled to two intracardiac leads for positioning electrodes in both anatrial and a ventricular chamber. Multi-chamber pacemakers are alsoavailable that may be coupled to three leads, for example, forpositioning electrodes for pacing and sensing in one atrial chamber andboth of the right and left ventricles.

Intracardiac pacemakers have been introduced that are implantable withina ventricular chamber of a patient's heart for delivering ventricularpacing pulses. Such a pacemaker may sense R-wave signals attendant tointrinsic ventricular depolarizations and deliver ventricular pacingpulses in the absence of sensed R-waves. While single chamberventricular sensing and pacing by an intracardiac ventricular pacemakermay adequately address some patient conditions, some patients maybenefit from atrial and ventricular (dual chamber) sensing for providingatrial-synchronized ventricular pacing in order to maintain a regularheart rhythm.

SUMMARY

The techniques of this disclosure generally relate to a pacemaker havinga motion sensor producing a motion signal, which may include signalsrepresentative of ventricular and atrial mechanical events. Thepacemaker is configured to sense atrial events, e.g., atrial mechanicalevent signals corresponding to atrial contractions that occur duringatrial systole, from the motion signal. The sensed atrial events may beused for controlling atrial synchronized ventricular pacing pulsesdelivered by the pacemaker in some examples. A pacemaker operatingaccording to the techniques disclosed herein adjusts sensing controlparameters used for sensing atrial event signals from the motion sensorsignal. The sensing control parameters may include one or more ofsensing window start times, sensing window end times, and/or sensingthreshold amplitudes applied to the motion sensor signal for sensing theatrial events.

In one example, the disclosure provides a medical device including amotion sensor and a control circuit. The motion sensor is configured toproduce a motion signal. The control circuit is configured to setsensing control parameters and sense atrial events from the motionsignal during multiple ventricular cycles according to the sensingcontrol parameters. The control circuit is configured to determine afeature of the motion signal for each ventricular cycle of at least aportion of the multiple ventricular cycles, determine a metric of themotion signal based on the determined features, and adjust at least oneof the sensing control parameters based on the metric. The controlcircuit is configured to sense an atrial event from the motion signalaccording to the adjusted sensing control parameter and generate anatrial sensed event signal in response to sensing the atrial event.

In another example, the disclosure provides a method including producinga motion signal by a motion sensor, setting multiple sensing controlparameters and sensing atrial events from the motion signal duringmultiple ventricular cycles according to the sensing control parameters.The method may include determining a feature of the motion signal foreach ventricular cycle of at least a portion of the multiple ventricularcycles, determining a metric of the motion signal based on thedetermined features, adjusting at least one of the sensing controlparameters based on the metric and sensing an atrial event from themotion signal according to the adjusted sensing control parameter. Themethod may include generating an atrial sensed event signal in responseto sensing the atrial event according to the adjusted sensing controlparameter.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control circuit of a medical device, cause themedical device to produce a motion signal by a motion sensor, setmultiple sensing control parameters, sense atrial events from the motionsignal during multiple ventricular cycles according to the sensingcontrol parameters. The instructions further cause the device todetermine a feature of the motion signal for each ventricular cycle ofat least some of the multiple ventricular cycles, determine a metric ofthe motion signal based on the determined features, adjust at least oneof the sensing control parameters based on the metric, and sense anatrial event from the motion signal according to the adjusted sensingcontrol parameter. The instructions may cause the device to generate anatrial sensed event signal in response to sensing the atrial eventaccording to the adjusted sensing control parameter.

The details of one or more aspects of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the techniques described in this disclosurewill be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating a medical device system thatmay be used to sense cardiac electrical signals and motion signalsinduced by cardiac motion and flowing blood and provide pacing therapyto a patient's heart.

FIG. 2 is a conceptual diagram of the intracardiac pacemaker shown inFIG. 1 .

FIG. 3 is a schematic diagram of an example configuration of thepacemaker shown in FIG. 1 .

FIG. 4 is an example of a motion sensor signal that may be produced by amotion sensor over a cardiac cycle.

FIG. 5 is an example of motion sensor signals produced over twodifferent cardiac cycles.

FIG. 6 is a flow chart of a method for adjusting atrial event sensingcontrol parameters according to some examples.

FIG. 7 is a diagram illustrating a method for determining a controlparameter metric during an atrial tracking ventricular pacing modeaccording to one example.

FIG. 8 is a diagram of four ventricular cycles representing variousscenarios that may occur during ventricular cycles over which a controlparameter metric is being determined.

FIG. 9 is a flow chart of a method for determining control parametermetrics for use in adjusting an atrial event low sensing thresholdaccording to one example.

FIG. 10 is a flow chart of a method for determining an adjustment to theatrial event low sensing threshold amplitude according to one example.

FIG. 11 is a flow chart of an illustrative method for determining whenatrial event low sensing threshold adjustment criteria are met accordingto one example.

FIG. 12 is a diagram of an electrogram (EGM) signal, a motion sensorsignal and various features of the motion sensor signal that may bedetermined for determining A4 sensing control parameter metrics.

FIG. 13 is a flow chart of a method performed by a medical device fordetermining A4 sensing control parameter metrics and adjusting a sensingwindow ending time according to one example.

FIG. 14 is a flow chart of a method that may be performed by a medicaldevice at for determining when sensing window ending time adjustmentcriteria are met.

FIG. 15 is a flow chart of a method performed by a medical device foradjusting an atrial event high sensing threshold amplitude according toone example.

FIG. 16 is a flow chart 900 of a method that may be performed by controlcircuit 206 for adjusting the high A4 sensing threshold amplitudeaccording to another example.

FIG. 17 is a diagram 950 of histograms of A3 event amplitudes that maybe determined by control circuit 206 and stored in memory 210 for use insetting the high A4 sensing threshold amplitude according to someexamples.

FIG. 18 is a flow chart of a method for adjusting atrial event sensingcontrol parameters from starting values to operational values accordingto one example.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for adjusting cardiacevent sensing parameters by an implantable medical device. As describedbelow, a motion sensor signal, such as an accelerometer signal, mayinclude cardiac event signals attendant to the mechanical contractionand relaxation (and filling) of a heart chamber. Cardiac event signalsmay be sensed from a signal produced by the motion sensor. The motionsensor signal may include cardiac event signals corresponding toventricular events and atrial events. For example, an atrial systolicevent signal corresponding to atrial mechanical contraction and theactive filling phase of the ventricle, sometimes referred to as the“atrial kick” may be present in a motion sensor signal implanted in aventricular chamber. The atrial event may be sensed from the motionsensor signal using atrial event sensing control parameters, which mayinclude one or more sensing threshold amplitudes and/or time windows.The atrial event may be sensed according to sensing control parametersfrom among other cardiac event signals that may occur in the motionsensor signal during a cardiac cycle. The techniques disclosed hereinprovide techniques for sensing cardiac events, e.g., atrial events, froma motion sensor signal and adjusting the cardiac event sensing controlparameters to improve cardiac event sensing performance by a medicaldevice.

In some examples, the medical device is a ventricular pacemaker, whichmay be wholly implantable within a ventricular heart chamber, having amotion sensor for producing an intraventricular motion signal. Atrialsystolic events can be sensed from within the ventricle from the motionsensor signal for use in controlling atrial synchronized ventricularpacing, for example. Atrial-synchronized ventricular pacing pulses canbe delivered by a pacemaker implanted in the ventricle without requiringa sensor in or on the atria of the patient's heart for sensing atrialevents.

FIG. 1 is a conceptual diagram illustrating an implantable medicaldevice (IMD) system 10 that may be used to sense cardiac electricalsignals and cardiac mechanical signals induced by cardiac motion andflowing blood and provide pacing therapy to a patient's heart 8. IMDsystem 10 includes a ventricular intracardiac pacemaker 14. Pacemaker 14may be a transcatheter intracardiac pacemaker which is adapted forimplantation wholly within a heart chamber, e.g., wholly within theright ventricle (RV) or wholly within the left ventricle (LV) of heart 8for sensing cardiac signals and delivering ventricular pacing pulses.Pacemaker 14 may be reduced in size compared to subcutaneously implantedpacemakers and may be generally cylindrical in shape to enabletransvenous implantation via a delivery catheter.

Pacemaker 14 is shown positioned in the RV, along an endocardial wall,e.g., near the RV apex though other locations are possible. Thetechniques disclosed herein are not limited to the pacemaker locationshown in the example of FIG. 1 and other positions within heart 8 arepossible. For example, ventricular intracardiac pacemaker 14 may bepositioned in the LV and configured to detect cardiac motion signals anddeliver atrial-synchronized ventricular pacing to the LV using thetechniques disclosed herein. Pacemaker 14 may be positioned within theRV or LV to provide respective right ventricular or left ventricularpacing and for sensing cardiac mechanical event signals from a signalproduced by a motion sensor within the ventricular chamber. In otherexamples, pacemaker 14 is not necessarily required to be implantedinside a heart chamber and may be positioned outside the heart, e.g.,along the RV or LV in an epicardial location.

Pacemaker 14 is capable of producing electrical stimulation pulses,e.g., pacing pulses, delivered to heart 8 via one or more electrodes onthe outer housing of the pacemaker. Pacemaker 14 is configured todeliver RV pacing pulses and sense an RV cardiac electrical signal usinghousing based electrodes for producing an RV electrogram (EGM) signal.The cardiac electrical signals may be sensed using the housing basedelectrodes that are also used to deliver pacing pulses to the RV in someexamples.

Pacemaker 14 is configured to control the delivery of ventricular pacingpulses to the RV in a manner that promotes synchrony between atrialactivation and ventricular activation, e.g., by maintaining a targetatrioventricular (AV) interval between atrial events and ventricularpacing pulses. That is, pacemaker 14 controls pacing pulse delivery tomaintain a desired AV interval between atrial contractions correspondingto atrial systole and ventricular pacing pulses delivered to causeventricular depolarization and ventricular systolic contractions.

According to the techniques described herein, atrial systolic eventsproducing the active ventricular filling phase are detected by pacemaker14 from a motion sensor signal such as an accelerometer signal,generated by a motion sensor that may be enclosed by the housing ofpacemaker 14. The motion signal produced by an accelerometer implantedwithin a ventricular chamber, which may be referred to as an“intraventricular motion signal,” includes motion signals caused byventricular and atrial events. For example, acceleration of bloodflowing into the RV through the tricuspid valve 16 between the RA and RVcaused by atrial systole, and referred to as the “atrial kick,” may bedetected by pacemaker 14 from the signal produced by an accelerometerincluded in pacemaker 14. Other motion signals that may be detected bypacemaker 14, such as motion caused by ventricular contraction andpassive ventricular filling are described below in conjunction with FIG.4 .

Atrial P-waves that are attendant to atrial depolarization arerelatively low amplitude signals in the near-field ventricular cardiacelectrical signal received by pacemaker 14 (e.g., compared to thenear-field R-wave attendant to ventricular depolarization) and thereforecan be difficult to reliably detect from the cardiac electrical signalacquired by pacemaker 14 when implanted in a ventricular chamber.Atrial-synchronized ventricular pacing by pacemaker 14 or otherfunctions that rely on atrial sensing may not be reliable when basedsolely on a cardiac electrical signal received by pacemaker 14.According to the techniques disclosed herein, pacemaker 14 includes amotion sensor, such as an accelerometer, and is configured to detect anatrial event corresponding to atrial mechanical activation or atrialsystole from a signal produced by the motion sensor. Ventricular pacingpulses may be synchronized to the atrial event that is detected from themotion sensor signal by setting a programmable AV pacing interval thatcontrols the timing of the ventricular pacing pulse relative to thedetected atrial systolic event. As described below, detection of theatrial systolic event used to synchronize ventricular pacing pulses toatrial systole may include detection of other cardiac event motionsignals in order to positively identify the atrial systolic event and/oradjust atrial systolic event sensing control parameters.

A target AV interval may be a default value or a programmed valueselected by a clinician and is the time interval from the detection ofthe atrial event until delivery of the immediately subsequentventricular pacing pulse. In some instances, the target AV interval maybe started from the time the atrial systolic event is detected based ona motion sensor signal or starting from an identified fiducial point ofthe atrial event signal. The target AV interval may be identified asbeing hemodynamically optimal for a given patient based on clinicaltesting or assessments of the patient or based on clinical data from apopulation of patients. The target AV interval may be determined to beoptimal based on relative timing of electrical and mechanical events asidentified from the cardiac electrical signal received by pacemaker 14and the motion sensor signal received by pacemaker 14. The AV intervalmay be programmed to 10 to 50 ms, as examples.

Pacemaker 14 may be capable of bidirectional wireless communication withan external device 20 for programming the AV pacing interval and otherpacing control parameters as well as cardiac event sensing parameters,which may be utilized for detecting ventricular mechanical events andthe atrial systolic event from the motion sensor signal. Aspects ofexternal device 20 may generally correspond to the externalprogramming/monitoring unit disclosed in U.S. Pat. No. 5,507,782(Kieval, et al.), hereby incorporated herein by reference in itsentirety. External device 20 is often referred to as a “programmer”because it is typically used by a physician, technician, nurse,clinician or other qualified user for programming operating parametersin pacemaker 14. External device 20 may be located in a clinic, hospitalor other medical facility. External device 20 may alternatively beembodied as a home monitor or a handheld device that may be used in amedical facility, in the patient's home, or another location. Operatingparameters, including sensing and therapy delivery control parameters,may be programmed into pacemaker 14 by a user interacting with externaldevice 20.

External device 20 may include a processor 52, memory 53, display unit54, user interface 56 and telemetry unit 58. Processor 52 controlsexternal device operations and processes data and signals received frompacemaker 14. Display unit 54 may generate a display, which may includea graphical user interface, of data and information relating topacemaker functions to a user for reviewing pacemaker operation andprogrammed parameters as well as cardiac electrical signals, cardiacmotion signals or other physiological data that may be acquired bypacemaker 14 and transmitted to external device 20 during aninterrogation session.

User interface 56 may include a mouse, touch screen, keypad or the liketo enable a user to interact with external device 20 to initiate atelemetry session with pacemaker 14 for retrieving data from and/ortransmitting data to pacemaker 14, including programmable parameters forcontrolling cardiac event sensing and therapy delivery. Telemetry unit58 includes a transceiver and antenna configured for bidirectionalcommunication with a telemetry circuit included in pacemaker 14 and isconfigured to operate in conjunction with processor 52 for sending andreceiving data relating to pacemaker functions via communication link24.

At the time of implant, during patient follow-up visits, or any timeafter pacemaker implantation, pacemaker 14 may perform a set-upprocedure to establish parameters used in detecting atrial events fromthe motion sensor signal. The patient may be standing, sitting, lyingdown or ambulatory during the process. The set-up procedure may includeacquiring motion sensor signal data and generating distributions ofmotion sensor signal features for establishing atrial event sensingparameters. Motion sensor signal data may be transmitted to externaldevice 20 and displayed on display unit 54 of external device 20 in theform of a histogram in some examples. The atrial event sensingparameters established based on the motion sensor signal data may be setautomatically or may be transmitted to external device 20 for generatinga display on display unit 54 as recommended parameters, allowing aclinician to review and accept or modify the recommended parameters,e.g., using user interface 56.

In some examples, external device processor 52 may execute operationsfor establishing a starting value of an atrial event sensing parameterbased on data retrieved from pacemaker 14. Processor 52 may causedisplay unit 54 to generate a display of data relating to a motionsensor signal, including histogram distributions of metrics determinedfrom a cardiac motion signal for use in selecting starting values ofatrial event sensing control parameters. Display unit 54 may be agraphical user interface that enables a user to interact with thedisplay, e.g., for selecting various displays or information forviewing. In some examples, a user may select one or more atrial eventsensing control parameter settings to be automatically established bypacemaker 14 and/or may program starting sensing control parameters orother programmable parameters for controlling sensor operation andtherapy delivery. In some examples, processing circuitry included inpacemaker 14 and/or processor 52 may determine starting values for oneor more atrial systolic event sensing control parameters based on dataacquired from acceleration signals produced by an accelerometer includedin pacemaker 14 and various thresholds and criteria, which may includeuser programmable thresholds or criteria used in setting the startingparameter values.

The starting value(s) of one or sensing control parameters may beadjusted by pacemaker 14 according to the techniques disclosed herein.After establishing a starting value of a sensing control parameter usedin sensing cardiac event signals from a cardiac motion signal, signalvariability due to various factors such as patient posture, patientphysical activity, patient heart rate, etc. may cause the starting valueof a sensing control parameter to no longer be optimal. In order topromote reliable cardiac mechanical event sensing from the motion signalover time, pacemaker 14 is configured to adjust sensing controlparameters. This automatic adjustment adapts and optimizes the sensingcontrol parameters as patient or signal sensing conditions change overtime, improving reliability of sensing of cardiac mechanical events anddelivery of pacing therapy, even in an ambulatory patient, and reducesthe burden on a clinician in manually adjusting sensing controlparameters.

External device telemetry unit 58 is configured for bidirectionalcommunication with implantable telemetry circuitry included in pacemaker14. Telemetry unit 58 establishes a wireless communication link 24 withpacemaker 14. Communication link 24 may be established using a radiofrequency (RF) link such as BLUETOOTH®, Wi-Fi, Medical ImplantCommunication Service (MICS) or other communication bandwidth. In someexamples, external device 20 may include a programming head that isplaced proximate pacemaker 14 to establish and maintain a communicationlink 24, and in other examples external device 20 and pacemaker 14 maybe configured to communicate using a distance telemetry algorithm andcircuitry that does not require the use of a programming head and doesnot require user intervention to maintain a communication link.

It is contemplated that external device 20 may be in wired or wirelessconnection to a communications network via a telemetry circuit thatincludes a transceiver and antenna or via a hardwired communication linefor transferring data to a centralized database or computer to allowremote management of the patient. Remote patient management systemsincluding a centralized patient database may be configured to utilizethe presently disclosed techniques to enable a clinician to review EGM,motion sensor signal, and marker channel data and authorize programmingof sensing and therapy control parameters in pacemaker 14, e.g., afterviewing a visual representation of EGM, motion sensor signal and markerchannel data.

FIG. 2 is a diagram of the intracardiac pacemaker 14 shown in FIG. 1according to one example. Pacemaker 14 includes electrodes 162 and 164spaced apart along the housing 150 of pacemaker 14 for sensing cardiacelectrical signals and delivering pacing pulses. Electrode 164 is shownas a tip electrode extending from a distal end 102 of pacemaker 14, andelectrode 162 is shown as a ring electrode along a mid-portion ofhousing 150, for example adjacent proximal end 104. Distal end 102 isreferred to as “distal” in that it is expected to be the leading end aspacemaker 14 is advanced through a delivery tool, such as a catheter,and placed against a targeted pacing site.

Electrodes 162 and 164 form an anode and cathode pair for bipolarcardiac pacing and sensing. In alternative embodiments, pacemaker 14 mayinclude two or more ring electrodes, two tip electrodes, and/or othertypes of electrodes exposed along pacemaker housing 150 for deliveringelectrical stimulation to heart 8 and sensing cardiac electricalsignals. Electrodes 162 and 164 may be, without limitation, titanium,platinum, iridium or alloys thereof and may include a low polarizingcoating, such as titanium nitride, iridium oxide, ruthenium oxide,platinum black, among others. Electrodes 162 and 164 may be positionedat locations along pacemaker 14 other than the locations shown.

Housing 150 is formed from a biocompatible material, such as a stainlesssteel or titanium alloy. In some examples, the housing 150 may includean insulating coating. Examples of insulating coatings include parylene,urethane, PEEK, or polyimide, among others. The entirety of the housing150 may be insulated, but only electrodes 162 and 164 uninsulated.Electrode 164 may serve as a cathode electrode and be coupled tointernal circuitry, e.g., a pacing pulse generator and cardiacelectrical signal sensing circuitry, enclosed by housing 150 via anelectrical feedthrough crossing housing 150. Electrode 162 may be formedas a conductive portion of housing 150 defining a ring electrode that iselectrically isolated from the other portions of the housing 150 asgenerally shown in FIG. 2 . In other examples, the entire periphery ofthe housing 150 may function as an electrode that is electricallyisolated from tip electrode 164, instead of providing a localized ringelectrode such as anode electrode 162. Electrode 162 formed along anelectrically conductive portion of housing 150 serves as a return anodeduring pacing and sensing.

The housing 150 may include a control electronics subassembly 152 and abattery subassembly 160, which provides power to the control electronicssubassembly 152. Control electronics subassembly 152 houses theelectronics for sensing cardiac signals, producing pacing pulses andcontrolling therapy delivery and other functions of pacemaker 14 asdescribed below in conjunction with FIG. 3 . A motion sensor may beimplemented as an accelerometer enclosed within housing 150 in someexamples. The accelerometer provides a signal to a processor included incontrol electronics subassembly 152 for signal processing and analysisfor detecting atrial systolic events, e.g., for use in controlling thetiming ventricular pacing pulses, as described below.

The accelerometer may be a three-dimensional accelerometer. In someexamples, the accelerometer may have one “longitudinal” axis that isparallel to or aligned with the longitudinal axis 108 of pacemaker 14and two orthogonal axes that extend in radial directions relative to thelongitudinal axis 108. Practice of the techniques disclosed herein,however, are not limited to a particular orientation of theaccelerometer within or along housing 150. In other examples, aone-dimensional accelerometer may be used to obtain an intracardiacmotion signal from which cardiac mechanical events are detected. Instill other examples, a two dimensional accelerometer or othermulti-dimensional accelerometer may be used. Each axis of a single ormulti-dimensional accelerometer may be defined by a piezoelectricelement, micro-electrical mechanical system (MEMS) device or othersensor element capable of producing an electrical signal in response tochanges in acceleration imparted on the sensor element, e.g., byconverting the acceleration to a force or displacement that is convertedto the electrical signal. In a multi-dimensional accelerometer, thesensor elements may be arranged orthogonally with each sensor elementaxis orthogonal relative to the other sensor element axes. Orthogonalarrangement of the elements of a multi-axis accelerometer, however, isnot necessarily required.

Each sensor element or axis may produce an acceleration signalcorresponding to a vector aligned with the axis of the sensor element. Avector signal of a multi-dimensional accelerometer (also referred to asa “multi-axis” accelerometer) for use in sensing cardiac mechanicalevents may be selected to include a single axis signal or a combinationof two or more axis signals. For example, one, two or all three axissignals produced by a three dimensional accelerometer may be selected asa vector signal for use in detecting atrial systolic events, e.g., forcontrolling atrial-synchronized ventricular pacing delivered bypacemaker 14. When two or more axis signals are selected, the axissignals may be summed or combined in another manner for producing amotion signal from which atrial systolic events may be sensed.

Pacemaker 14 may include features for facilitating deployment andfixation of pacemaker 14 at an implant site. For example, pacemaker 14may include a set of fixation tines 166 to secure pacemaker 14 topatient tissue, e.g., by actively engaging with the ventricularendocardium and/or interacting with the ventricular trabeculae. Fixationtines 166 are configured to anchor pacemaker 14 to position electrode164 in operative proximity to a targeted tissue for deliveringtherapeutic electrical stimulation pulses. Numerous types of activeand/or passive fixation members may be employed for anchoring orstabilizing pacemaker 14 in an implant position. Pacemaker 14 mayoptionally include a delivery tool interface 158. Delivery toolinterface 158 may be located at the proximal end 104 of pacemaker 14 andis configured to connect to a delivery device, such as a catheter, usedto position pacemaker 14 at an implant location during an implantationprocedure, for example within a heart chamber.

FIG. 3 is a conceptual diagram of an example configuration of pacemaker14 shown in FIG. 1 . Pacemaker 14 includes a pulse generator 202, acardiac electrical signal sensing circuit 204, a control circuit 206,memory 210, telemetry circuit 208, motion sensor 212 and a power source214. The various circuits represented in FIG. 3 may be combined on oneor more integrated circuit boards which include a specific integratedcircuit (ASIC), an electronic circuit, a processor (shared, dedicated,or group) and memory that execute one or more software or firmwareprograms, a combinational logic circuit, state machine or other suitablecomponents that provide the described functionality.

Motion sensor 212 may include an accelerometer in the examples describedherein. Motion sensor 212 is not limited to being an accelerometer,however, and other motion sensors may be utilized successfully inpacemaker 14 for detecting cardiac motion signals according to thetechniques described herein. Examples of motion sensors that may beimplemented in motion sensor 212 include piezoelectric sensors and MEMSdevices.

Motion sensor 212 may include a multi-axis sensor, e.g., atwo-dimensional or three-dimensional sensor, with each axis providing anaxis signal that may be analyzed individually or in combination fordetecting cardiac mechanical events. Motion sensor 212 produces anelectrical signal correlated to motion or vibration of sensor 212 (andpacemaker 14), e.g., when subjected to flowing blood and cardiac motion.The motion sensor 212 may include one or more filter, amplifier,rectifier, analog-to-digital converter (ADC) and/or other components forproducing a motion signal that is passed to control circuit 206. Forexample, each signal produced by each individual axis of a multi-axisaccelerometer may be filtered by a high pass filter, e.g., a 10 Hz highpass filter. The filtered signal may be digitized by an ADC andrectified for use by atrial event detector circuit 240 for detectingatrial systolic events. The high pass filter may be lowered (e.g., to 5Hz) if needed to detect atrial signals that have lower frequencycontent. In some examples, high pass filtering is performed with no lowpass filtering. In other examples, each accelerometer axis signal isfiltered by a low pass filter, e.g., a 30 Hz low pass filter, with orwithout high pass filtering.

One example of an accelerometer for use in implantable medical devicesthat may be implemented in conjunction with the techniques disclosedherein is generally disclosed in U.S. Pat. No. 5,885,471 (Ruben, etal.), incorporated herein by reference in its entirety. An implantablemedical device arrangement including a piezoelectric accelerometer fordetecting patient motion is disclosed, for example, in U.S. Pat. No.4,485,813 (Anderson, et al.) and U.S. Pat. No. 5,052,388 (Sivula, etal.), both of which patents are hereby incorporated by reference hereinin their entirety. Examples of three-dimensional accelerometers that maybe implemented in pacemaker 14 and used for detecting cardiac mechanicalevents using the presently disclosed techniques are generally describedin U.S. Pat. No. 5,593,431 (Sheldon) and U.S. Pat. No. 6,044,297(Sheldon), both of which are incorporated herein by reference in theirentirety. Other accelerometer designs may be used for producing anelectrical signal that is correlated to motion imparted on pacemaker 14due to ventricular and atrial events.

Sensing circuit 204 is configured to receive a cardiac electrical signalvia electrodes 162 and 164 by a pre-filter and amplifier circuit 220.Pre-filter and amplifier circuit 220 may include a high pass filter toremove DC offset, e.g., a 2.5 to 5 Hz high pass filter, or a widebandfilter having a passband of 2.5 Hz to 100 Hz to remove DC offset andhigh frequency noise. Pre-filter and amplifier circuit 220 may furtherinclude an amplifier to amplify the “raw” cardiac electrical signalpassed to analog-to-digital converter (ADC) 226. ADC 226 may pass amulti-bit, digital electrogram (EGM) signal to control circuit 206 foruse by atrial event detector circuit 240 in identifying ventricularelectrical events (e.g., R-waves or T-waves) and/or atrial electricalevents, e.g., P-waves. Identification of cardiac electrical events maybe used in algorithms for establishing atrial sensing control parametersand for detecting atrial systolic events from the motion sensor signal.The digital signal from ADC 226 may be passed to rectifier and amplifiercircuit 222, which may include a rectifier, bandpass filter, andamplifier for passing a cardiac signal to R-wave detector 224.

R-wave detector 224 may include a sense amplifier or other detectioncircuitry that compares the incoming rectified, cardiac electricalsignal to an R-wave sensing threshold, which may be an auto-adjustingthreshold. When the incoming signal crosses the R-wave sensingthreshold, the R-wave detector 224 produces an R-wave sensed eventsignal (R-sense) that is passed to control circuit 206. In otherexamples, R-wave detector 224 may receive the digital output of ADC 226for detecting R-waves by a comparator, morphological signal analysis ofthe digital EGM signal or other R-wave detection techniques. Processor244 may provide sensing control signals to sensing circuit 204, e.g.,R-wave sensing threshold, sensitivity, and various blanking andrefractory intervals applied to the cardiac electrical signal forcontrolling R-wave sensing. R-wave sensed event signals passed fromR-wave detector 224 to control circuit 206 may be used for schedulingventricular pacing pulses by pace timing circuit 242 and for use inidentifying the timing of ventricular electrical events in algorithmsperformed by atrial event detector circuit 240 for detecting atrialsystolic events from a signal received from motion sensor 212.

Control circuit 206 includes an atrial event detector circuit 240, pacetiming circuit 242, and processor 244. Control circuit 206 may receiveR-wave sensed event signals and/or digital cardiac electrical signalsfrom sensing circuit 204 for use in detecting and confirming cardiacevents associated with each heartbeat (e.g., intrinsic ventriculardepolarizations) and controlling ventricular pacing. For example, R-wavesensed event signals, which may correspond to intrinsic ventriculardepolarizations, may be passed to pace timing circuit 242 for inhibitingscheduled ventricular pacing pulses or for scheduling ventricular pacingpulses when pacemaker 14 is operating in a non-atrial trackingventricular pacing mode. R-wave sensed event signals may also be passedto atrial event detector circuit 240 for use in setting time windowsused by control circuit 206 in detecting atrial systolic events from themotion sensor signal.

Atrial event detector circuit 240 is configured to detect atrialsystolic events from a signal received from motion sensor 212.Techniques for setting time windows used in detecting atrial systolicevents are described below. In some examples, one or more ventricularmechanical events may be detected from the motion sensor signal in agiven cardiac cycle to facilitate positive detection of the atrialsystolic event from the motion sensor signal during the ventricularcycle and for adjusting atrial systolic event sensing controlparameters.

Atrial event detector circuit 240 receives a motion signal from motionsensor 212 and may start an atrial “blanking” period in response to aventricular electrical event, e.g., an R-wave sensed event signal fromsensing circuit 204 or delivery of a pacing pulse by pulse generator202. The blanking period may correspond to a time period after theventricular electrical event during which ventricular mechanical events,e.g., corresponding to ventricular contraction or opening and/or closingof semilunar valves, are expected to occur. When ventricular pacing isproperly synchronized to atrial events, an atrial event is not expectedto occur during the atrial blanking period, corresponding to ventricularsystole. The motion signal peaks that occur during the atrial blankingperiod, therefore, are not sensed as atrial events. The atrial“blanking” period may be used to define a time period following aventricular electrical event during which an atrial systolic event isnot sensed by atrial event detector circuit 240. The motion sensorsignal, however, is not necessarily blanked during this time period inthat control circuit 206 may still receive the motion sensor signalduring the atrial blanking period and may process the motion signal forsensing ventricular events during the atrial blanking period in someexamples.

For instance, the motion sensor signal during the blanking period may bemonitored by atrial event detector circuit 240 for the purposes ofdetecting ventricular mechanical events, which may be used forconfirming or validating atrial systolic event detection in someexamples. As such, ventricular mechanical event detection windows may beset during the atrial blanking period and may be set according topredetermined time intervals following identification of a ventricularelectrical event. Atrial event detector circuit 240 may be configured todetect one or more ventricular mechanical events during respectiveventricular event detection windows during the atrial blanking period.The timing and detection of the ventricular mechanical events may beused to update the atrial blanking period and/or may be used to confirmdetection of the atrial event occurring subsequent to expectedventricular mechanical events.

Atrial event detector circuit 240 determines if the motion sensor signalsatisfies atrial systolic event detection criteria outside of the atrialblanking period. In some examples, atrial event detector circuit 240 mayset an atrial refractory period starting upon a ventricular electricalevent, sensed or paced. Atrial event detector circuit 240 may set timewindows corresponding to the passive ventricular filling phase and theactive ventricular filling phase (atrial systole) based on the timing ofa preceding ventricular electrical event, either an R-wave sensed eventsignal or a ventricular pacing pulse. A motion sensor signal crossing ofan atrial event sensing threshold during either of these windows andoutside the atrial blanking period, may be detected as the atrialsystolic event. As described below, two different atrial event sensingthreshold values may be established for applying during the passivefilling phase window and after the passive filling phase window (duringan active filling phase window also referred to below as an “A4window”).

Atrial event detector circuit 240 may pass an atrial event detectionsignal to processor 244 and/or pace timing circuit 242 in response todetecting an atrial event. Pace timing circuit 242 (or processor 244)may additionally receive R-wave sensed event signals from R-wavedetector 224 for use in controlling the timing of pacing pulsesdelivered by pulse generator 202. Processor 244 may include one or moreclocks for generating clock signals that are used by pace timing circuit242 to time out an AV pacing interval that is started upon receipt of anatrial event detection signal from atrial event detector circuit 240. Insome examples, an atrial event sensed outside the atrial blanking periodbut within the atrial refractory period is classified as an atrialrefractory sense. An atrial refractory sense may not cause pace timingcircuit to start an AV pacing interval so that atrial refractory sensedevents are not tracked by ventricular pacing pulses. Atrial refractorysensed events may be used in some instances, however, e.g., foradjusting atrial event sensing control parameters or for determining anatrial rate.

Pace timing circuit 242 may include one or more pacing escape intervaltimers or counters that are used to time out the AV pacing interval,which may be a programmable interval stored in memory 210 and retrievedby processor 244 for use in setting the AV pacing interval used by pacetiming circuit 242. One application of atrial sensed event signalsproduced by atrial event detector circuit 240 is for setting AV pacingintervals for controlling the timing of ventricular pacing pulses.Control circuit 206, however, may use atrial sensed event signals forother purposes.

Pace timing circuit 242 may additionally include a lower pacing rateinterval timer for controlling a minimum ventricular pacing rate. Forexample, if an atrial systolic event is not detected from the motionsensor signal triggering a ventricular pacing pulse at the programmed AVpacing interval, a ventricular pacing pulse may be delivered by pulsegenerator 202 upon expiration of the lower pacing rate interval toprevent ventricular asystole and maintain a minimum ventricular rate. Attimes, control circuit 206 may control pulse generator 202 in anon-atrial tracking ventricular pacing mode (which may also be referredto as “asynchronous ventricular pacing”) during a process forestablishing sensing parameters used for detecting atrial systolicevents from the motion signal. The non-atrial tracking ventricularpacing mode may be denoted as a VDI pacing mode in which ventricularpacing pulses are delivered in the absence of a sensed R-wave andinhibited in response to an R-wave sensed event signal from sensingcircuit 204. Dual chamber sensing may be performed during the non-atrialtracking ventricular pacing mode by sensing ventricular electricalevents by sensing circuit 204 and sensing atrial systolic events fromthe motion signal received by atrial event detector circuit 240 frommotion sensor 212. Atrial event sensing parameters established during aVDI pacing mode may include the end time of a passive ventricularfilling window, the atrial event sensing threshold amplitude valuesapplied during and after the passive ventricular filling window, and thepost-ventricular atrial refractory period, as examples. These atrialevent sensing parameters established during a VDI pacing mode may beapplied during a VDD pacing mode in which the ventricular pacing pulsestrack sensed atrial events at the AV interval.

Pulse generator 202 generates electrical pacing pulses that aredelivered to the RV of the patient's heart via cathode electrode 164 andreturn anode electrode 162. In addition to providing control signals topace timing circuit 242 and pulse generator 202 for controlling thetiming of ventricular pacing pulses, processor 244 may retrieveprogrammable pacing control parameters, such as pacing pulse amplitudeand pacing pulse width, which are passed to pulse generator 202 forcontrolling pacing pulse delivery. Pulse generator 202 may includecharging circuit 230, switching circuit 232 and an output circuit 234.

Charging circuit 230 may include a holding capacitor that may be chargedto a pacing pulse amplitude by a multiple of the battery voltage signalof power source 214 under the control of a voltage regulator. The pacingpulse amplitude may be set based on a control signal from controlcircuit 206. Switching circuit 232 may control when the holdingcapacitor of charging circuit 230 is coupled to the output circuit 234for delivering the pacing pulse. For example, switching circuit 232 mayinclude a switch that is activated by a timing signal received from pacetiming circuit 242 upon expiration of an AV pacing interval (or VV lowerrate pacing interval) and kept closed for a programmed pacing pulsewidth to enable discharging of the holding capacitor of charging circuit230. The holding capacitor, previously charged to the pacing pulsevoltage amplitude, is discharged across electrodes 162 and 164 throughthe output capacitor of output circuit 234 for the programmed pacingpulse duration. Examples of pacing circuitry generally disclosed in U.S.Pat. No. 5,507,782 (Kieval, et al.) and in U.S. Pat. No. 8,532,785(Crutchfield, et al.), both of which patents are incorporated herein byreference in their entirety, may be implemented in pacemaker 14 forcharging a pacing capacitor to a predetermined pacing pulse amplitudeunder the control of control circuit 206 and delivering a pacing pulse.

Memory 210 may include computer-readable instructions that, whenexecuted by control circuit 206, cause control circuit 206 to performvarious functions attributed throughout this disclosure to pacemaker 14.The computer-readable instructions may be encoded within memory 210.Memory 210 may include any non-transitory, computer-readable storagemedia including any volatile, non-volatile, magnetic, optical, orelectrical media, such as a random access memory (RAM), read-only memory(ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM(EEPROM), flash memory, or other digital media with the sole exceptionbeing a transitory propagating signal. Memory 210 may store timingintervals and other data used by control circuit 206 to control thedelivery of pacing pulses by pulse generator 202, e.g., by detecting anatrial event by atrial event detector circuit 240 from the motion sensorsignal according to the techniques disclosed herein and setting a pacingescape interval timer included in pace timing circuit 242. Memory 210may include various buffers for storing threshold crossing times and/oramplitudes of the motion sensor signal for use in adjusting atrial eventsensing control parameters as described below.

Power source 214 provides power to each of the other circuits andcomponents of pacemaker 14 as required. Power source 214 may include oneor more energy storage devices, such as one or more rechargeable ornon-rechargeable batteries. The connections between power source 214 andother pacemaker circuits and components are not shown in FIG. 3 for thesake of clarity but are to be understood from the general block diagramof FIG. 3 . For example, power source 214 may provide power as needed tocharging and switching circuitry included in pulse generator 202,amplifiers, ADC 226 and other components of sensing circuit 204,telemetry circuit 208, memory 210, and motion sensor 212.

Telemetry circuit 208 includes a transceiver 209 and antenna 211 fortransferring and receiving data via a radio frequency (RF) communicationlink. Telemetry circuit 208 may be capable of bi-directionalcommunication with external device 20 (FIG. 1 ) as described above.Motion sensor signals and cardiac electrical signals, and/or dataderived therefrom may be transmitted by telemetry circuit 208 toexternal device 20. Programmable control parameters and algorithms forperforming atrial event detection and ventricular pacing control may bereceived by telemetry circuit 208 and stored in memory 210 for access bycontrol circuit 206.

The functions attributed to pacemaker 14 herein may be embodied as oneor more processors, controllers, hardware, firmware, software, or anycombination thereof. Depiction of different features as specificcircuitry is intended to highlight different functional aspects and doesnot necessarily imply that such functions must be realized by separatehardware, firmware or software components or by any particular circuitarchitecture. Rather, functionality associated with one or more circuitsdescribed herein may be performed by separate hardware, firmware orsoftware components, or integrated within common hardware, firmware orsoftware components. For example, atrial systolic event detection fromthe motion sensor signal and ventricular pacing control operationsperformed by pacemaker 14 may be implemented in control circuit 206executing instructions stored in memory 210 and relying on input fromsensing circuit 204 and motion sensor 212. Providing software, hardware,and/or firmware to accomplish the described functionality in the contextof any modern pacemaker, given the disclosure herein, is within theabilities of one of skill in the art.

FIG. 4 is an example of a motion sensor signal 250 that may be acquiredby motion sensor 212 over a cardiac cycle. Vertical dashed lines 252 and262 denote the timing of two consecutive ventricular electrical events(an intrinsic ventricular depolarization or a ventricular pacing pulse),marking the respective beginning and end of the ventricular cycle 251.The motion signal includes an A1 event 254, an A2 event 256, an A3 event258 and an A4 event 260. The A1 event 254 is an acceleration signal (inthis example when motion sensor 212 is implemented as an accelerometer)that occurs during ventricular contraction and marks the approximateonset of ventricular mechanical systole. The A1 event is also referredto herein as a “ventricular contraction event.” The A2 event 256 is anacceleration signal that may occur with closure of the aortic andpulmonic valves, marking the approximate offset or end of ventricularmechanical systole. The A2 event may also mark the isovolumic relaxationphase of the ventricles that occurs with aortic and pulmonic valveclosure.

The A3 event 258 is an acceleration signal that occurs during passiveventricular filling and marks ventricular mechanical diastole. The A3event is also referred to herein as and as a “ventricular diastolicevent.” Since the A2 event occurs with the end of ventricular systole,it may be an indicator of the onset of ventricular diastole. The A3event occurs during ventricular diastole. As such, the A2 and A3 eventsmay be collectively referred to as ventricular mechanical diastolicevents because they are both indicators of the ventricular diastolicperiod.

The A4 event 260 is an acceleration signal that occurs during atrialcontraction and active ventricular filling and marks atrial mechanicalsystole. The A4 event 260 is also referred to herein as the “atrialevent” that is detected from motion sensor signal 250. Atrial eventdetector circuit 240 detects A4 event 260 using atrial event sensingcontrol parameters as disclosed herein. Processor 244 may control pacetiming circuit 242 to trigger a ventricular pacing pulse by starting anAV pacing interval in response to detecting the A4 event 260, e.g., whenit occurs outside a post-ventricular atrial blanking period and after apost-ventricular atrial refractory period. Control circuit 206 may beconfigured to detect one or more of the A1, A2, and A3 events frommotion sensor signal 250, for at least some ventricular cardiac cycles,for use in positively detecting the A4 event 260 and setting atrialevent detection control parameters. The A1, A2 and/or A3 events may bedetected and characterized to avoid false detection of A4 events andpromote reliable A4 event detection for proper timing ofatrial-synchronized ventricular pacing pulses in some examples.

Techniques described below in conjunction with flow charts and diagramspresented herein may be performed by pacemaker 14 for adjusting sensingcontrol parameters used for sensing A4 events, without necessarilyrequiring identification and discrimination of the A1-A4 events. Themotion signal may be characterized by determining features of the motionsignal during and after a passive ventricular filling window, outside anatrial blanking period. These features are used in establishing atrialevent sensing parameters by control circuit 206.

FIG. 5 is an example of motion sensor signals 400 and 410 acquired overtwo different cardiac cycles. A ventricular pacing pulse is delivered attime 0.0 seconds for both cardiac cycles. The top motion sensor signal400 is received over one cardiac cycle, and the bottom motion sensorsignal 410 is received over a different cardiac cycle. The two signals400 and 410 are aligned in time at 0.0 seconds, the time of theventricular pacing pulse delivery. While motion signals 400 and 410 andmotion signal 250 of FIG. 4 are shown as raw accelerometer signals, itis recognized that control circuit 206 may receive a digitized filtered,amplified and rectified signal from motion sensor 212 for processing andanalysis as described in conjunction with the accompanying drawings.

The A1 events 402 and 412 of the respective motion sensor signals 400and 410, which occur during ventricular contraction, are observed to bewell-aligned in time following the ventricular pacing pulse at time 0.0seconds. Similarly, the A2 events 404 and 414 (which may mark the end ofventricular systole and the isovolumic ventricular relaxation phase) andthe A3 events 406 and 416 (occurring during passive ventricular filling)are well-aligned in time. Since the A1, A2 and A3 events are ventricularevents, occurring during ventricular contraction, at the end ofventricular systole and during passive ventricular filling,respectively, these events are expected to occur at relativelyconsistent intervals following a ventricular electrical event, theventricular pacing pulse in this example, and relative to each other.The time relationship of the A1, A2 and A3 events may be differentfollowing a ventricular pacing pulse compared to following a sensedintrinsic R-wave, however, during a stable paced or intrinsicventricular rhythm, the relative timing of ventricular A1, A2 and A3events to each other and the immediately preceding ventricularelectrical event is expected to be consistent from beat-to-beat.

The A4 events 408 and 418 of the first and second motion sensor signals400 and 410 respectively are not aligned in time. The A4 event occursduring atrial systole and as such the time interval of the A4 eventfollowing the immediately preceding ventricular electrical event (sensedR-wave or ventricular pacing pulse) and the preceding A1 through A3events may vary between cardiac cycles.

The consistency of the timing of the A1 through A3 events relative toeach other and the immediately preceding ventricular electrical eventmay be used for determining an atrial blanking period 436 and increasingconfidence in reliably detecting A4 events 408 and 418. The atrialsystolic event is not detected during the atrial blanking period 436which may extend from the ventricular electrical event (at time 0.0)through an estimated onset of ventricular diastole so that the atrialblanking period 436 includes both the A1 and A2 events in some examples.An A3 window 424 may be set having a starting time 420 corresponding tothe end of the post-ventricular atrial blanking period 436 and having anending time 422. The ending time 422 may be adjusted using techniquesdescribed herein, e.g., below in conjunction with FIGS. 12-14 . Theending time 422 may also be considered a starting time of an A4 sensingwindow 450, though A4 events may be sensed during the A3 window in someinstances. The A3 window 424 is also referred to here as a “ventriculardiastolic event window,” and ending time 422 is also referred to hereinas a “ventricular diastolic event window ending time” since the A3 eventcorresponding to passive ventricular filling, a ventricular diastolicevent, is expected to occur during the A3 window, before ending time422.ear

A4 events 408 and 418 may be detected based on a multi-level A4 sensingthreshold 444. As seen by the lower motion sensor signal 410, the A4event 418 may occur earlier after the A3 window 424 due to changes inatrial rate. In some instances, as the atrial rate increases, the A4event 418 may occur within the A3 window 424. When this occurs, the A3event 416 and the A4 event 418 may fuse as passive and activeventricular filling occur together. The fused A3/A4 event may have ahigh amplitude, even greater than the amplitude of either the A3 event416 or the A4 event 418 when they occur separately. As such, in someexamples a first, higher A4 sensing threshold amplitude 446 may beestablished for detecting an early A4 event that is fused with the A3event during the A3 window 424. A second, lower A4 sensing thresholdamplitude 448 may be established for detecting relatively later A4events, after the ending time 422 of the A3 window 424, during an A4window 450. The A4 window 450 extends from the ending time 422 of the A3window 424 until the next ventricular electrical event, sensed or paced.The earliest crossing of the A4 sensing threshold 444 by the motionsensor signal after the starting time 420 of the A3 window (or after theexpiration of the atrial blanking period 436) may be detected as theatrial systolic event. Atrial event detector circuit 240 may sense theA4 event in response to the earliest crossing time of the high A4sensing threshold amplitude or the low A4 sensing threshold amplitude.Techniques for adjusting the high A4 sensing threshold amplitude 446used during the A3 window 424 and the low A4 sensing threshold amplitude448 used after the ending time 422 of the A3 window 424, during the A4window 450, are described below in conjunction with FIGS. 7-15 .

In some examples, control circuit 206 may set a post-ventricular atrialrefractory period (PVARP) 438. The PVARP 438 may extend from theventricular electrical event (sensed R-wave or ventricular pacing pulse)for a time interval longer than the post-ventricular atrial blankingperiod 436. Depending on the end time 422 of the A3 window 424, thePVARP 438 will generally expire during the A3 window 424. When themotion sensor signal crosses the high A4 sensing threshold amplitude 446during the PVARP 438, but outside the blanking period 436, a refractoryA4 sense may be made by atrial event detector circuit 240. Pace timingcircuit 242 does not set an AV pacing interval in response to arefractory A4 sense, but control circuit 206 may use the refractory A4sense in adjusting A4 sensing control parameters. When the motion sensorsignal crosses the A4 sensing threshold 444 after the expiration ofPVARP 438, atrial event detector circuit 240 senses the atrial systolicevent, and pacing timing circuit 242 starts an AV pacing interval (notshown in FIG. 5 ). Upon expiration of the AV pacing interval, pulsegenerator 202 generates a pacing pulse delivered to the ventricle totrack the non-refractory sensed atrial event.

FIG. 6 is a flow chart 300 of a method for adjusting atrial eventsensing control parameters according to some examples. Control circuit206 may perform the method of flow chart 300 to automatically adjustvalues of atrial event sensing control parameters, also referred toherein as “A4 sensing control parameters,” used in sensing A4 eventsfrom the motion sensor signal. In some examples, the A4 events aresensed during an atrial tracking ventricular pacing mode, e.g., a VDDpacing mode. The VDD pacing mode may be a temporary pacing mode or apermanent pacing mode. For example, pacemaker 14 may be operating in apermanent VDD pacing mode that temporarily switches to a non-atrialtracking ventricular pacing mode, e.g., a VDI(R) or VVI(R) pacing mode.When control circuit 206 is not operating in an atrial trackingventricular pacing mode, as determined at block 302, adjustments to oneor more atrial event sensing control parameters may be disabled.

In some instances, pacemaker 14 may be operating in a temporary VDIpacing mode at block 302, e.g., upon implantation of pacemaker 14, toallow starting values of the atrial sensing control parameters to bedetermined and adjusted to values that are tailored to the patient. Oneexample of an A4 sensing control parameter adjustment process isdescribed below in conjunction with FIG. 18 . Starting values of theatrial event sensing control parameters may be set according touser-programmed or default, nominal values. In other examples, startingvalues of the atrial event sensing control parameters may be establishedduring an automatic set-up process performed by pacemaker 14 duringwhich motion signal features are determined over a period of time togenerate distributions and/or median values or other metrics of thesignal features. Starting atrial event sensing control parameters may beestablished based on the generated distributions and median values.Example techniques for establishing starting values of atrial eventsensing control parameters are generally disclosed in commonly assignedU.S. Patent Application Publication No. 2020/0179707 (Splett, et al.)and U.S. Patent Application Publication No. 2020/0179708 (Splett, etal.), both of which are incorporated herein by reference in theirentirety. As described in conjunction with FIG. 18 , the starting valuesof at least some A4 sensing control parameters may be adjusted during anon-atrial tracking ventricular pacing mode that includes atrial andventricular (dual chamber) sensing, e.g., a VDI pacing mode.

When control circuit 206 is operating in a specified pacing mode atblock 302 (e.g., VDI or VDD), control circuit 206 may enable and performadjustments to one or more A4 sensing control parameters. In someexamples, automatic adjustment of an A4 sensing control parameter may beenabled or disabled by a user, e.g., via a programming commandtransmitted by external device 20 and received by telemetry circuit 208.Adjustment of A4 sensing control parameters may be programmably enabledor disabled individually or as a group. As such in some examples,control circuit 206 may determine at block 302 that the automaticadjustment of a given A4 sensing control parameter is programmed “on” orenabled by a user and that the current pacing mode is a designatedpacing mode during which A4 sensing control parameter adjustments can beperformed.

At block 304, control circuit 206 determines sensing control parametermetrics from the motion signal over a predetermined number of cardiaccycles. The sensing control parameter metrics are features orcharacteristics of the motion sensor signal that are determined bycontrol circuit 206 for use in setting or adjusting A4 sensing controlparameters that discriminate the A4 event (which may be fused with theA3 event) from the A1, A2 and A3 events. As described in detail below,the sensing control parameter metrics may be determined based on themotion sensor signal amplitude, e.g., a maximum peak amplitude or apredetermined threshold amplitude, during a specified time window of theventricular cycle. Control circuit 206 may adjust the A4 sensing controlparameters based on the control parameter metrics determined from themotion sensor signal over a predetermined number of ventricular cycles.Some control parameter metrics are features or characteristics of themotion sensor signal that provide information about the expected timingand/or amplitude of the atrial systolic event to enable A4 sensingcontrol parameters to be adjusted appropriately. Other control parametermetrics are features or characteristics of the motion sensor signal thatprovide information about the expected timing and/or amplitude of otherfeatures of the motion signal, e.g., A3 event signals, which may enablecontrol circuit 206 to adjust A4 sensing control parameters in a mannerthat avoids oversensing of A3 event signals or other motion signals thatare not the A4 event signal.

In the illustrative examples presented herein, the sensing controlparameter metrics are determined from a predetermined number ofconsecutive ventricular cycles, for example 8 consecutive ventricularcycles. In other examples, sensing control parameter metrics may bedetermined from a predetermined number of ventricular cycles thatinclude an A4 window. The A4 window may not be started in a givenventricular cycle when an A4 event is sensed during the A3 window or anintrinsic R-wave is sensed before the A4 window. The term “applicablecycle” is used herein to refer to ventricular cycles that include an A4window that is started after expiration of the A3 window ending time. Insome examples, the sensing control parameter metrics are determined froma predetermined number of applicable cycles, e.g., 8 applicable cycles,which may or may not be consecutive ventricular cycles.

In various examples, the determination of A4 sensing control parametermetrics over one or more ventricular cycles of the motion signal mayinclude determining the maximum amplitude of the motion signal duringthe A4 window, the time of the maximum amplitude during the A4 windowrelative to a most recent preceding ventricular electrical event, themaximum amplitude of the motion signal during the A3 window, and alatest time of a low threshold amplitude crossing during the A3 window.Other control parameter metrics that may be determined may relate to thenumber or count of ventricular cycles during which an A4 event is sensedduring the A4 window, the number or count of ventricular cycles duringwhich an A4 event is sensed during the A3 window, the number or count ofventricular cycles with an A4 window (e.g., the number of cycles thatare longer than the ending time of the A3 window), the number or countof ventricular cycles having an A4 event sensed early during the A4window, the number or count of ventricular cycles having an A4 eventsensed late during the A4 window or other parameters counted over apredetermined number of ventricular cycles. Example methods fordetermining the control parameter metrics are described below, e.g., inconjunction with FIGS. 7-8 . The control parameter metrics may bedetermined from one or more ventricular cycles as a mean, median, aspecified ordinal (nth) highest or lowest value, or a frequency or countof a specified event out of the multiple ventricular cycles.

At block 306, control circuit 206 may verify that other adjustmentcriteria are met before performing an A4 sensing control parameteradjustment based on the control parameter metrics determined at block304. Various criteria may require that a sensing control parameter bewithin a predetermined range, e.g., less than or equal to a maximumadjustable value or greater than or equal to a minimum adjustable value,before adjusting the sensing control parameter. Other criteria mayverify that an A4 event is detected for a minimum number of thepredetermined number of ventricular cycles such that a given controlparameter metric determined at block 304 over the predetermined numberof ventricular cycles is considered a valid metric to base an adjustmenton. Other criteria may require that the A4 sensing control parametermetrics determined from the motion sensor signal at block 304 indicate achange in the atrial systolic event timing and/or amplitude and/or achange in the timing and/or amplitude of the A3 event. As furtherdescribed below, control circuit 206 may determine that adjustmentcriteria are met by comparing the A4 sensing control parameter metricsdetermined at block 304 to various criteria for detecting evidence of A4event undersensing and/or evidence of oversensing of A3 events or othermotion signals as false A4 events.

When adjustment criteria are unmet, as determined by control circuit 206at block 306, A4 sensing control parameter adjustment may not beperformed (or the adjustment is set to zero). Control circuit 206returns to block 302. When adjustment criteria are met at block 306,control circuit 206 determines an adjustment value at block 308. Asdescribed below, the A4 sensing control parameter metrics determinedfrom the motion sensor may be evaluated to determine if a given A4sensing control parameter should be incremented or decremented from itscurrent value. In some examples, a target value of an A4 sensing controlparameter may be determined at block 308 based on the A4 sensing controlparameter metrics determined at block 304 so that the A4 sensing controlparameter may be appropriately incremented or decremented toward thetarget value or left unchanged.

At block 310, control circuit 206 adjusts the A4 sensing controlparameter according to the determined adjustment. The A4 sensing controlparameter may be the A3 window ending time (e.g., ending time 422 inFIG. 5 ), the high A4 threshold amplitude applied during the A3 window(e.g., high A4 threshold 446 in FIG. 5 ), the low A4 sensing thresholdamplitude applied after the A3 window (e.g., low A4 threshold 448 inFIG. 5 ), and/or the PVARP (e.g., PVARP 438 shown in FIG. 5 ). Examplemethods performed by control circuit 206 for determining A4 sensingcontrol parameter metrics from the motion sensor signal (at block 304),determining if adjustment criteria are met (at block 306), anddetermining the appropriate adjustment to be made (block 308) foradjusting each of the example A4 sensing control parameters listed aboveare described below in conjunction with the accompanying flow charts anddiagrams.

FIG. 7 is a diagram 320 illustrating a method for determining an A4sensing control parameter metric from the motion sensor signal during anatrial tracking ventricular pacing mode according to one example. Inthis example, eight ventricular cycles are illustrated, each startingwith a ventricular pacing pulse (VP) 322, tracked to a preceding A4sensed event signal 328 by an AV pacing interval 340. The end of theeight ventricular cycles is indicated by 332, which is the final, endingVP of the group of eight cycles. The AV pacing interval 340 is thepacing interval set by pace timing circuit 242 in response to the A4sensed event signal 328 and extends from the A4 sensed event signal 328to the subsequently ventricular pacing pulse 322 delivered by pulsegenerator 202 upon expiration of the AV pacing interval 340.

In response to each VP 322, control circuit 206 starts an A3 window 324according to a predetermined A3 window starting time, e.g., 600 ms afterthe VP 322. The A4 window 326 starts upon the expiration of the A3window 324 at an A3 window ending time 325 and extends until an A4signal 330 is sensed or a ventricular pacing rate interval (VV interval)expires resulting in a ventricular pacing pulse when an A4 signal is notsensed. In the example of FIG. 7 , an A4 event 330 is sensed in responseto a crossing of the low A4 sensing threshold amplitude 338 (during theA4 window 326) during each of the eight ventricular cycles shown. An A4sensed event signal 328 may be generated by atrial event detectorcircuit 240 in response to the earliest A4 sensing threshold amplitudecrossing during each ventricular cycle (outside the post-ventricularblanking period). The next VP 322 is generated by pulse generator 202upon expiration of the AV pacing interval 340 started by pace timingcircuit 242 in response to the A4 sensed event signal 328.

It is noted that the durations of the A3 window 324, the A4 window 326and the AV pacing interval 340 are shown for illustration purposes andnot necessarily shown according to time scale relative to one another orthe ventricular cycle lengths as a whole in the diagram 320. Forexample, the AV pacing interval 340 may be 10 to 30 ms (e.g., 20 ms)while the A3 window 324 may be 200 to 300 ms long, and the A4 window 326may be even longer, e.g., 400 ms or longer in some examples. Therelative durations of the A3 windows 324, A4 windows 326 and AV pacingintervals 340 are shown for the sake of illustration and convenience inFIG. 7 with no limitations intended.

Control circuit 208 may determine an A4 sensing control parameter metricevery eight (or other selected number) ventricular cycles, e.g., uponthe ending VP 332 of the eight ventricular cycles shown in FIG. 7 . Inthe example shown in FIG. 7 , the A4 sensing control parameter metricbeing determined by control circuit 206 is the minimum detected A4amplitude. The maximum motion signal amplitude during the A4 window 326may be determined as the A4 signal amplitude during each ventricularcycle that includes an A4 sensed event signal 328 after the A3 windowending time 326. The lowest maximum amplitude value 334 determinedduring the eight ventricular cycles is determined upon expiration of theeight ventricular cycles (at VP 332) as the minimum detected A4amplitude. As described below this minimum detected A4 amplitude is afeature of the motion sensor signal determined over the specified numberof ventricular cycles by control circuit 206 as an A4 sensing controlparameter metric used in adjusting the low A4 sensing thresholdamplitude 338.

For example, if this minimum detected A4 amplitude 334 is at least 1.5times the low A4 sensing threshold amplitude 338, the low A4 sensingthreshold amplitude 338 may be increased. An adequate safety marginbetween the minimum detected A4 amplitude 334 and the low A4 sensingthreshold amplitude 338 exists, such that control circuit 206 mayincrease the low A4 sensing threshold amplitude 338. When controlcircuit 206 determines that the peak amplitudes of the sensed A4 eventsare relatively high (based on the minimum detected A4 amplitude 334) andA4 events are being regularly sensed during most ventricular cycles(e.g., at least 75% of ventricular cycles), the low A4 sensing thresholdamplitude 338 may be incremented to avoid oversensing of other motionsignals (which may be cardiac or body motion or other signal noise)while still providing a reasonable safety margin between the minimumdetected A4 amplitude 334 and the low A4 sensing threshold amplitude338.

Other multiples of the low A4 sensing threshold amplitude 338 may becompared to the minimum detected A4 amplitude 334 to determine if morethan a minimum desired safety margin exists between the low A4 sensingthreshold amplitude 338 and the minimum detected A4 amplitude,justifying an increase in the low A4 sensing threshold amplitude. Inother examples, 1.25 times the low A4 sensing threshold amplitude 338 upto 2 times the low A4 sensing threshold amplitude 338 may be used as anadjustment threshold applied to the minimum detected A4 amplitude 334.When the minimum detected A4 amplitude 334 is at least the selectedmultiple greater than the current value of the low A4 sensing thresholdamplitude 338, the low A4 sensing threshold amplitude 338 may be safelyincreased by control circuit 206. A relatively lower multiple, like 1.25times the low A4 sensing threshold amplitude 338, tends to allow controlcircuit 206 to increment the low A4 sensing threshold amplitude 338 moreoften and toward higher values while maintaining a relatively lowersafety margin between the low A4 sensing threshold amplitude 338 and theminimum detected A4 amplitude 334. A relatively higher multiple, like 2times the low A4 sensing threshold amplitude 338, tends to cause controlcircuit 206 to increment the low A4 sensing threshold amplitude lessoften, holding the low A4 sensing threshold amplitude to relativelylower values over time, with a greater safety margin between the low A4sensing threshold amplitude 338 and the minimum detected A4 amplitude334.

FIG. 8 is a diagram 350 of four ventricular cycles, labeled I, II, III,and IV, representing various scenarios that may occur during the eight(or other predetermined number) ventricular cycles over which an A4sensing control parameter metric is being determined from the motionsensor signal. In some examples, each ventricular cycle may beclassified by control circuit 206 as an “applicable” cycle or a“non-applicable” cycle depending on whether an A4 window is started ornot. Control circuit 206 may determine whether adjustment criteria aremet, e.g., at block 306 of FIG. 6 , based on the number of applicablecycles during a consecutive series of the predetermined number ofventricular cycles.

In the example of FIG. 7 , an A4 signal was detected during the A4window 326 during each ventricular cycle. This may not always be thecase, however. For instance, in the first ventricular cycle I of FIG. 8, a VP 352 is followed by an A3 window 354 and an A4 window 356, howeverthe motion sensor signal 351 does not cross the high A4 sensingthreshold 376 or the low A4 sensing threshold 368 before the ventricularpacing interval (VV interval) 358 expires. The VV interval 358 may be aprogrammed lower ventricular rate interval or may be adjusted from thelower ventricular rate interval to a rate smoothing interval based on arecently determined actual ventricular rate. AVP 362 is generated bypulse generator 202 upon expiration of the VV interval 358. Since an A4signal is not sensed, the A4 event amplitude cannot be determined duringthe first ventricular cycle I. However, the ventricular cycle I may beconsidered an applicable cycle for the purposes of determining when A4sensing control parameter adjustment criteria are met (e.g., at block306 of FIG. 6 ). The A4 sensing control parameter adjustment criteriamay require that the A4 window is started for at least a thresholdnumber of the predetermined number of ventricular cycles, for example.The first ventricular cycle I may be determined to be an applicablecycle by control circuit 206 since an A4 window 356 was started, eventhough an A4 signal was not sensed.

The second ventricular cycle II starts with VP 362 and ends with VP 372,which is generated by pulse generator 202 at an AV pacing interval 370from an A4 sensed event signal 365. The A4 window 366 is started at theending time 325 of the A3 window, and the A4 signal 360 is sensed duringthe A4 window 366 due to the motion signal crossing the low A4 sensingthreshold amplitude 368. The second ventricular cycle II is counted asan applicable cycle by control circuit 206 because an A4 window 366 isstarted. The maximum amplitude of the motion sensor signal during the A4window 366 may be determined by control circuit 206 for use indetermining the minimum detected A4 amplitude over a predeterminednumber of ventricular cycles as one A4 sensing control parameter metricdetermined from the motion sensor signal by control circuit 206.

The third ventricular cycle III starts upon VP 372 and ends upon VP 382.In this cycle, the A4 signal 380 crosses the high A4 sensing threshold376 during the A3 window 374. As such an A4 window is never started. TheVP 382 is delivered at an AV pacing interval 370 from the A4 sensedevent signal 375. Since an A4 window is not started, the thirdventricular cycle III may be classified as a non-applicable cycle bycontrol circuit 206 for purposes of determining an applicable cyclecount and applying criteria for adjusting an A4 sensing controlparameter and/or for determining an adjustment applied to the A4 sensingcontrol parameter. An applicable cycle may be any ventricular cycle thatis longer than the A3 window ending time 325.

In some examples, control circuit 206 extends the A3 window ending time325 when the A4 event is sensed during the A3 window 374, as inventricular cycle III. Control circuit 206 starts AV pacing interval 370which may expire after the normally scheduled A3 window ending time 325.As such, control circuit 206 may extend the A3 window ending time fromscheduled ending time 325 to an extended ending time 327 that occurssimultaneously with the expiration of the AV pacing interval 370, sothat no A4 window is started. In this case, the extended A3 windowending time 327 expires with the AV pacing interval 270 that was startedduring the A3 window 374 but may end later than the normal A3 windowending time 325 when the A4 event is sensed late in the A3 window 374.The extended A3 window ending time 327 ends with the ventricular pacingpulse 382, which precludes starting the A4 window so that theventricular cycle III is not counted as an applicable cycle.

The fourth ventricular cycle IV starts upon VP 382 and ends on aventricular sensed event 392. In this example, the A3 window 384 isstarted, but an intrinsic R-wave, which may be a premature ventricularcontraction (PVC), is sensed by cardiac electrical signal sensingcircuit 204 as shown by ventricular sensed event signal (VS) 392 duringthe A3 window 384, before an A4 window is started. The fourthventricular cycle IV is classified as a non-applicable cycle by controlcircuit 206 because an A4 window is not started.

Accordingly, in some examples, each ventricular cycle of thepredetermined number of ventricular cycles, e.g., eight, over which anA4 sensing control parameter metric is determined from the motion signalmay be classified as either “applicable” when an A4 window is started(whether or not the A4 signal is sensed) or “non-applicable” when an A4window is not started. In some examples, adjustment criteria at block306 of FIG. 6 may require a minimum number of applicable cycles to bereached during each set of N ventricular cycles. When fewer than thethreshold number of applicable cycles are classified and counted bycontrol circuit 206, the adjustment criteria at block 306 may bedetermined to be unmet since insufficient A4 signal amplitude and timinginformation may be available.

As in FIG. 7 , the durations of the A3 windows, A4 windows, AV pacingintervals 370 and ventricular cycles I, II, III and IV shown in FIG. 8are illustrative and conceptual in nature and not intended to belimiting. The relative durations of the A3 windows, A4 windows, AVpacing intervals and overall ventricular cycles may not be shownaccording to a relative time scale to one another. For example, an A4window 356 that is started in ventricular cycle I and ends with aventricular pacing pulse 362 at the VV pacing interval 358 will tend tobe a relatively long A4 window (when no A4 event is sensed) compared toan A4 window 366 during which the A4 event is sensed, as in ventricularcycle II. The A4 windows may be longer than or shorter than the A3windows, depending on when and if the A4 event is sensed. As such, it isnoted that the relative time durations of the A3 windows and A4 windowsmay vary from cycle to cycle and may be different relative to oneanother, the ventricular cycle lengths and the AV pacing interval thanas shown in the examples of FIG. 8 .

FIG. 9 is a flow chart 500 of a method for determining A4 sensingcontrol parameter metrics by control circuit 206 for use in adjustingthe low A4 sensing threshold according to one example. In some examples,the A4 sensing threshold is automatically adjusted by control circuit206 only during an atrial tracking ventricular pacing mode, e.g., a VDDpacing mode, which may be a temporary or permanent VDD pacing mode. Whenthe pacing mode is a non-atrial tracking pacing mode, e.g., VDI(R) orVVI(R), adjustments of the low A4 sensing threshold may be disabled orsuspended. As shown in FIG. 5 , the low A4 sensing threshold, e.g.,threshold 448 is the threshold amplitude applied to the motion sensorsignal after the A3 window ending time for sensing A4 events. If thepacing mode is an atrial tracking pacing mode at block 502, controlcircuit 206 determines the A4 event amplitude and time at block 504 foreach A4 event that is sensed during the A4 window following eachventricular pacing pulse or sensed R-wave for a predetermined number ofventricular cycles, e.g., eight cycles. The predetermined number ofventricular cycles may be consecutive cycles, e.g., as shown in FIG. 7 ,but A4 events may or may not be sensed in the A4 window of each of theconsecutive ventricular cycles.

For each A4 event that is sensed over the N ventricular cycles, the A4event amplitude may be determined as the maximum peak amplitude of themotion sensor signal after the low A4 sensing threshold crossing thatoccurs later than the A3 window ending time (and before the nextventricular electrical event). The A4 event time may be determined asthe sample number or time stamp of the maximum peak amplitude since themost recent preceding ventricular electrical event (either the mostrecent preceding ventricular pacing pulse or sensed R-wave).

Control circuit 206 continues to determine the A4 event amplitude andtime for each A4 event sensed during an A4 window (block 504) until Nventricular cycles have elapsed (block 506). In some cases, an A4 eventmay not be sensed during the A4 window for each of N ventricular cyclesor an A4 window may not occur (due to an early sensed R-wave or an earlysensed A4 event during the A3 window) as described in conjunction withFIG. 8 above. As such, in some examples, after N ventricular cycles haveelapsed, up to N A4 event amplitudes and A4 event times, but sometimesfewer than N A4 event amplitudes and A4 event times, may be determinedby control circuit 206 and buffered in memory 210 for the N ventricularcycles.

Once N ventricular cycles have elapsed (“yes branch of block 506),control circuit 206 may determine one or more A4 sensing controlparameter metrics based on the motion sensor signal over the Nventricular cycles. At block 508, control circuit 206 may determine theapplicable cycle count. The applicable cycle count is the number ofcycles out of the N ventricular cycles in which the A4 window is startedas described above in conjunction with FIG. 8 . A buffer in memory 210may store a flag for each ventricular cycle that includes an A4 windowduring the N ventricular cycles to facilitate determination of theapplicable cycle count at block 508. Even though an A4 window is startedduring a ventricular cycle, an A4 event may or may not be sensed duringthe A4 window. Another buffer in memory 210 may store a flag for eachventricular cycle of the N ventricular cycles during which an A4 eventis sensed during the A4 window to facilitate determination of a detectedA4 count at block 510. Control circuit 206 may count the number of flagsin the buffer that are set in response to each A4 sensed event signalgenerated by atrial systolic event detector circuit 240 during an A4window over the N ventricular cycles.

At block 511, the minimum detected A4 amplitude is determined as thelowest value of A4 event amplitudes determined at block 504 over the Nventricular cycles. At block 512, control circuit 206 may determine anearly A4 count. An A4 event sensed during the A4 window within apredetermined threshold time interval of the ending time of the A3window is classified as an early A4 event. For instance, each A4 sensedevent signal that occurs within 50 ms (or other selected time interval)of the ending time of the A3 window may be counted as an early A4 event.Control circuit 206 may subtract the A3 window ending time from the A4event time determined at block 504 and compare the difference to athreshold time interval, e.g., 50 ms. If the difference is less than thethreshold time interval, an early A4 event flag may be stored in abuffer in memory 206 for the corresponding ventricular cycle. Controlcircuit 206 may determine the early A4 count at block 512 by countingthe number of early A4 event flags set over the N ventricular cycles.

At block 514, control circuit 206 determines a normal A4 count bycounting the number of A4 sensed event signals generated by atrialsystolic event detector circuit 240 during the A4 window at or laterthan the early event threshold time interval. For example, controlcircuit 206 may set a flag in a buffer for each ventricular cycle duringwhich an A4 sensed event signal is generated at 50 ms or more after thenending time of the A3 window. Control circuit 206 may count the numberof normal A4 event flags set over the N ventricular cycles at block 514.

Control circuit 206 determines the A3 window detection count at block518. Each time an A4 event is sensed during the A3 window, in responseto the motion sensor signal crossing the high A4 sensing thresholdamplitude, the A4 event may be fused or overlapping with the A3 event orthe A3 event may be oversensed as an A4 event. In some instances,atrioventricular synchrony may be lost due to a loss in regular A4 eventsensing, which may occur when the A3 event is oversensed as an A4 event,before the A4 signal actually occurs during the ventricular cycle. Aloss of atrial tracking by the ventricular pacing pulses may occur inconjunction with A3 event oversensing, as evidenced by frequent A4sensed event signals generated by atrial systolic event detector circuit240 during the A3 window and/or early after the A3 window ending time.The ventricular pacing pulses may be tracking oversensed A3 eventsinstead of true A4 events. In this situation, decreasing the low A4sensing threshold amplitude during the A4 window may contribute to A3event oversensing with the associated loss in atrial tracking of theventricular pacing pulses. As such, for each ventricular cycle duringthe N ventricular cycles that an A4 sensed event signal is generated byatrial systolic event detector circuit 240 during the A3 window, controlcircuit 206 may set an A3 window detection flag in a buffer in memory210 to facilitate determining an A3 window detection count at block 518.This count of A4 events sensed in A3 windows may be used in verifyingconditions of A3 event oversensing and/or possible A4 event undersensingthat is occurring due to A3 event oversensing.

At block 520, control circuit 206 may determine whether the low A4sensing threshold amplitude should be adjusted and, if so, determinesthe adjustment to be made based on the various A4 sensing controlparameter metrics determined at blocks 508-518 over the N ventricularcycles. For example, when the currently set low A4 sensing thresholdamplitude does not exceed an upper adjustment limit and a thresholdnumber of early A4 events are counted, the low A4 sensing thresholdamplitude may be increased. When the minimum detected A4 amplitude isless than or equal to a predetermined factor of the currently set low A4sensing threshold amplitude, the low A4 sensing threshold may bedecreased to restore a desired safety margin between the minimumdetected A4 amplitude and the low A4 sensing threshold amplitude. Whenthe minimum detected A4 amplitude is greater than a predetermined factorof the currently set low A4 sensing threshold amplitude, the low A4sensing threshold may be increased. Various criteria based on the A4sensing control parameter metrics that may be applied at block 520 indetermining whether to increment the low A4 sensing threshold, decrementthe low A4 sensing threshold or leave the low A4 sensing thresholdunchanged are described below in conjunction with FIG. 10 .

When adjustment criteria are met and the appropriate increment ordecrement is determined at block 520, control circuit 206 adjusts thelow A4 sensing threshold amplitude at block 522. In some examples, thelow A4 sensing threshold amplitude is increased or decreased by apredetermined increment or decrement, respectively, when specifiedconditions are met. In other examples, the low A4 sensing thresholdamplitude may be adjusted to a value that is determined based on one ormore of the control parameter metrics. For example, the low A4 sensingthreshold amplitude may be adjusted to a value that is set based on theminimum detected A4 amplitude. For instance, the low A4 sensingthreshold amplitude may be adjusted directly to a fraction or percentageof the minimum detected A4 amplitude to achieve a desired safety marginrather than being adjusted by a predetermined increment or decrement.

After adjusting the low A4 sensing threshold amplitude at block 522,control circuit 206 may return to block 502 to proceed with determiningthe A4 sensing control parameter metrics for the next series of Nventricular cycles as long as the atrial tracking pacing mode remains ineffect (as determined at block 502). In some examples, the pacing modemay only be switched on every eight ventricular cycles, concurrentlywith the determination of the A4 sensing control parameter metrics andany resulting A4 sensing control parameter adjustments. As such, if apacing mode switch occurs, e.g., to a rate responsive pacing mode or anon-atrial tracking ventricular pacing mode, the automatic adjustment ofthe low A4 sensing threshold amplitude may be suspended or disableduntil control module 206 switches back to the atrial trackingventricular pacing mode. In other examples, the pacing mode may switchduring the N ventricular cycles. When this occurs, control circuit 206may suspend and disable the A4 sensing threshold amplitude adjustmentwithout determining A4 sensing control parameter metrics after N cyclesif the new pacing mode is not an atrial tracking ventricular pacingmode.

Furthermore, while the flow chart 500 (and other flow charts presentedherein) refer to some A4 sensing control parameters being adjusted afterevery N cycles (based on A4 sensing control parameter metrics determinedfrom the N cycles), it is to be understood that the adjustment may besuspended for a predetermined interval of time or number of ventricularcycles between the sets of N cycles to provide periodic adjustment ofone or more A4 sensing control parameters. The number of ventricularcycles between each set of N cycles may be greater than, less than orequal to N. To illustrate, the low A4 sensing threshold (or another A4sensing control parameter) may be adjusted after 8 ventricular cycles.Control circuit 206 may suspend adjustments for the next 8 to 16 cycles,then adjust the low A4 sensing threshold again after the next 8ventricular cycles. In this way the low A4 sensing threshold amplitudemay be adjusted based on the most recent 8 ventricular cycles but isadjusted every 16 to 24 cycles, as an example.

FIG. 10 is a flow chart 520 of a method for determining an adjustment tothe low A4 sensing threshold amplitude according to one example. Theprocess of flow chart 520 may correspond to the determination performedby control circuit 206 at like-numbered block 520 of FIG. 9 . As such,once an adjustment decision is made at block 538 (decrement), block 544(increment) or block 546 (no adjustment) according to the process offlow chart 520 as described below, control circuit 206 may advance toblock 522 of FIG. 9 to make the determined adjustment accordingly. Inthe process of flow chart 520, various conditions relating to the timingand/or amplitude of motion signal features and/or sensed A4 events maybe analyzed by control circuit 206, based on an analysis of the A4sensing control parameter metrics determined as described above inconjunction with FIG. 9 . When specified conditions/criteria are met,control circuit 206 may determine that an adjustment, e.g., apredetermined increment or decrement, is to be applied at block 522 ofFIG. 9 .

For example, control circuit 206 may be configured to detect evidence ofA3 event oversensing, or other motion signal oversensing, causing falseA4 sensed event signals to be generated by atrial systolic eventdetector circuit 240. At block 530, control circuit 206 may detectevidence of A3 event oversensing when early A4 window sensing conditionsare met. Control circuit 206 may determine that generated A4 sensedevent signals are occurring early in the A4 window with a relativelyhigh frequency. The low A4 sensing threshold amplitude applied duringthe A4 window may be lower than optimal when A4 events are being sensedrelatively quickly after the A3 window ending time, e.g., within 50 to100 ms after the A3 window ending time. When this occurs, the A4 eventsignal may actually be occurring later in the A4 window but due to a lowvalue of the low A4 sensing threshold amplitude, A3 events or othermotion signals may be oversensed as A4 events early in the A4 windowbefore the true A4 event signal. Accordingly, control circuit 206 mayexamine the A4 sensing control parameter metrics to determine if A4sensed event signals are frequently occurring relatively early in the A4window at block 530, indicating possible A3 event or other motion signaloversensing.

Various criteria may be applied at block 530 by control circuit 206 todetermine if A4 event signals are being sensed early in the A4 windowsuch that early A4 window sensing conditions are met, suggesting thatthe A3 event signal, or the end of the A3 event signal, or otherbaseline motion signal noise is being sensed as the A4 event early afterthe A3 window ending time. In one example, control circuit 206 comparesthe early A4 count determined at block 512 of FIG. 9 to an early A4count threshold value. For example, if the early A4 count (number of A4sensed event signals generated within 50 ms or other predetermined timeinterval after A3 window ending time) is at least 1, 2, 3, 4 or otherselected number out of (or percentage of) the N ventricular cycles,evidence of possible A3 event or other motion signal oversensing may bedetected based at least on early A4 window sensing conditions being metat block 530.

Control circuit 206 may additionally or alternatively compare the earlyA4 count to the normal A4 count. The normal A4 count determined at block514 (FIG. 9 ) is the number of A4 events sensed later than thepredetermined time interval after the A3 window ending time, e.g., at 50ms or later after the A3 window ending time. When the early A4 count isat least equal to or greater than the normal A4 count, control circuit206 may detect evidence of frequent A3 event or other motion signaloversensing leading to false A4 sensed event signals being generatedearly in the A4 window.

In some examples, a combination of conditions may be required to be met,e.g., requiring a threshold value of the early A4 count and the early A4count being greater than or equal to the normal A4 count, for controlcircuit 206 to determine that early A4 window sensing conditions are metat block 530. In an illustrative example, control circuit 206 maydetermine that the early A4 window sensing conditions are met at block530 when the early A4 count is at least 25% of the N ventricular cycles(e.g., 2 out of eight ventricular cycles) and the early A4 count isgreater than or equal to the normal A4 count. In other examples, one ormore amplitude conditions may be required to be met at block 530 thatwould indicate that false A4 event sensing may be occurring. Forexample, if the maximum motion sensor signal amplitude during the A4window occurs more than a threshold time interval after the low A4sensing threshold crossing time (and the A4 sensed event signal isgenerated) for each of the early A4 events counted, this amplitudecondition may indicate premature A4 event signal sensing due tooversensing of the A3 event signal and/or other motion signals.

Various early A4 window sensing conditions may be applied by controlcircuit 206 at block 530. Such criteria may be time-based and/oramplitude-based criteria. For example, control circuit 206 may comparethe time of sensed A4 events (e.g., the time of the low A4 sensingthreshold amplitude crossing during the A4 window) to early A4 windowsensing criteria, compare the time from the low A4 sensing thresholdamplitude crossing to a maximum motion sensor signal amplitude duringthe A4 window to early A4 window sensing criteria, compare thedifference between the maximum motion sensor signal amplitude during theA4 window and the low A4 sensing threshold amplitude to early A4 windowsensing criteria, and/or other timing and/or amplitude related criteriathat are indicative of probable premature sensing of a non-A4 eventduring the A4 window, e.g., due to A3 event oversensing early in the A4window. When early A4 window sensing conditions/criteria are met,control circuit 206 may determine if criteria for incrementing the lowA4 threshold are met at block 532.

Control circuit 206 may apply the increment criteria at block 532 inorder to avoid increasing the low A4 sensing threshold above a maximumlimit, and/or increasing the low A4 sensing threshold that is already ata relatively high value. For example, the adjusted low A4 sensingthreshold amplitude value may be limited to be no more than a low A4threshold maximum limit, which may correspond to an acceleration of 5m/s² or other selected upper limit. It is to be understood that examplesof acceleration thresholds or maximum and minimum values given hereinmay be implemented in ADC units in atrial event detector circuit 240.For instance, each ADC unit may correspond to 11.8 milli-g (where 1 g isthe acceleration of gravity) and 100 milli-g may correspond to 1 m/s²acceleration. When control circuit 206 determines that adjusting the lowA4 sensing threshold by a predetermined increment would cause the low A4sensing threshold amplitude to exceed the maximum limit, the incrementcriteria at block 532 are unmet. Control circuit 206 may determine thatno adjustment should be made at block 546.

Additionally or alternatively, control circuit 206 may compare thecurrent value of the low A4 sensing threshold amplitude to an adjustmentthreshold at block 532. For instance, the current value of the low A4sensing threshold amplitude may be required to be less than or equal totwice (or other multiple of) a minimum limit of the low A4 sensingthreshold amplitude. To illustrate, if the minimum available setting ofthe low A4 sensing threshold amplitude is 0.8 m/s², the currently setlow A4 sensing threshold amplitude may be required to be equal to orless than twice the minimum setting, or 1.6 m/s² in this example for theincrement criteria to be met at block 532. If the currently set low A4sensing threshold amplitude is greater than the adjustment threshold,control circuit 206 may determine that the increment criteria are notmet at block 532, and a determination of no adjustment is made at block546.

In other examples, control circuit 206 may compare the current value ofthe low A4 sensing threshold amplitude to one half (or other fractionof) the maximum available setting of the low A4 sensing thresholdamplitude. Generally, the control circuit 206 may determine that theincrement criteria are met a block 532 when the current setting of thelow A4 sensing threshold amplitude is relatively low in the range ofavailable settings, e.g., in the lower half of the range of availablesettings.

In still other examples, control circuit 206 may additionally oralternatively compare the current value of the low A4 sensing thresholdamplitude to one or more previous values of the low A4 sensing thresholdamplitude. The maximum (or minimum) allowable setting of the low A4sensing threshold amplitude may be based on one or more historicalvalues of the low A4 sensing threshold amplitude. For instance, the lowA4 sensing threshold amplitude may be limited to a maximum value that isset to a percentage of the maximum, mean or median of low A4 sensingthreshold amplitude settings that may be buffered in memory 210, e.g.,the most recent four, eight, twelve or other recent values. In otherexamples, the historical values of the A4 sensing threshold amplitudesettings may be stored since the time of implant and start of operationof pacemaker 14.

When control circuit 206 determines that the early A4 window sensingconditions are met at block 530 and the increment criteria are met atblock 532, control circuit 206 may determine that the low A4 sensingthreshold amplitude is to be adjusted by a predetermined increment ofADC units, e.g., one or more ADC units, which may correspond to anacceleration increment of 0.1 m/s², 0.2 m/s², 0.25 m/s², 0.3 m/s², orother predetermined acceleration increment. Control circuit 206 may setthe adjustment increment at block 544 to be used for adjusting the lowA4 sensing threshold amplitude at block 422 of FIG. 9 . The incrementmay be a fixed increment, e.g., 0.3 m/s², that is used any time early A4window sensing conditions are met at block 530 and the incrementcriteria are met at block 532. It is to be understood that theadjustment may be a portion of a predetermined increment when increasingby a whole increment would exceed a maximum upper limit of the low A4sensing threshold amplitude.

In other examples the increment set at block 544 may be an adjustableincrement that may be relatively larger or smaller depending on thecurrent value of the low A4 sensing threshold amplitude, the values ofthe A4 sensing control parameter metrics and conditions met at block530. For example, when the early A4 count is relatively high or muchgreater than the normal A4 count and/or the current value of the low A4sensing threshold amplitude is at or near the minimum limit, arelatively higher increment may be set at block 544. If the early A4count is relatively low or equal to the normal A4 count, or the currentvalue of the low A4 sensing threshold amplitude is near a maximum limitor in a higher range of available settings, a relatively lower incrementmay be set at block 544. In this way, the adjustment increment set atblock 544 may be scaled based on one or more factors, such as thecurrent value of the low A4 sensing threshold amplitude, the frequencyof A4 events that are sensed early in the A4 window and/or thedifference between the current value of the low A4 sensing thresholdamplitude and the minimum detected A4 amplitude during the A4 windowsover the N ventricular cycles.

When the early A4 window sensing conditions are not met at block 530(“no” branch of block 530), control circuit 206 may determine ifconditions/criteria are met that indicate possible undersensing of theA4 event signal during the A4 window. When the A4 event is not sensed inthe A3 window, so that an A4 window is started at the A3 window endingtime, and an A4 event signal is not detected before the next ventricularpacing pulse or sensed R-wave, the A4 event signal may be undersenseddue to the low A4 sensing threshold amplitude being set too high.Accordingly, control circuit 206 may compare one or more of the A4sensing control parameter metrics determined over the N ventricularcycles to criteria for detecting evidence of A4 event undersensingduring the A4 window at block 534.

In one example, control circuit 206 compares the detected A4 count(determined at block 510 of FIG. 9 ) to the applicable cycle count (thenumber of ventricular cycles during which the A3 window ends and an A4window is started as determined at block 508 of FIG. 9 ). When thedetected A4 count is less than the applicable cycle count by more than athreshold difference (or percentage), control circuit 206 may detectevidence of undersensing in the A4 window at block 534. Variouscomparisons may be made by control circuit 206 to determine if thenumber of A4 events sensed during the A4 windows that occur over the Nventricular cycles is relatively low, indicating possible undersensingof the A4 event signal. For example, a ratio of the detected A4 count tothe applicable cycle count may be compared to a threshold ratio orpercentage. Control circuit 206 may detect evidence of A4 undersensingwhen the detected A4 count is less than 100%, 90%, 80%, 75%, 70% orother selected percentage of the applicable cycle count.

In some examples, the applicable cycle count may be required to begreater than or equal to a specified threshold, e.g., at least 3, 4, 5or other selected number (or percentage) of the N ventricular cycles inorder to detect evidence of A4 undersensing during the A4 window.Control circuit 206 may determine that a minimum threshold number of A4windows are started (upon the A3 window ending before the end of theventricular cycle) over the N ventricular cycles in order for theconditions to be met for detecting evidence of A4 undersensing at block534. When a small portion of the N ventricular cycles have an A4 windowstarted, e.g., only one out of eight ventricular cycles or less than 20%(or other threshold percentage) of the N ventricular cycles, there maybe insufficient evidence to detect likely A4 event undersensing.Conditions that may result in no A4 window being started during aventricular cycle are described above in conjunction with FIG. 8 . Insome examples, control circuit 206 determines whether the number ofapplicable cycles over the N ventricular cycles is less than 50%, 40%,30%, 25%, 20%, or 15% of the N ventricular cycles in order for criteriato be met for determining A4 undersensing during the A4 window at block534.

Control circuit 206 may determine that evidence of A4 event undersensingcriteria are met at block 534 when the detected A4 count is less than athreshold percentage of, or difference from, the applicable cycle countand the applicable cycle count is at least a threshold count. Forinstance, when at least five out of eight ventricular cycles areapplicable cycles (have an A4 window started) and an A4 event is sensedduring fewer than two less than the number of applicable cycles,criteria for detecting evidence of undersensing of the A4 event duringthe A4 windows may be met at block 534. Additionally or alternatively,control circuit 206 may compare the normal A4 count to a threshold atblock 534. When a threshold number of applicable cycles occur but nonormal A4 events are sensed, which are A4 events sensed at least apredetermined time interval, e.g., 50 ms, after the A3 window endingtime, A4 event undersensing during the A4 window may be occurring, whichmay be due to a low A4 threshold amplitude that is set too high.

In still other examples, control circuit 206 may compare the minimumdetected A4 amplitude (determined at block 511 of FIG. 9 ) to athreshold, which may be based on the current setting of the low A4sensing threshold amplitude. When the minimum detected A4 amplitude isless than a threshold amplitude, the likelihood of A4 event undersensingmay be increased due to an insufficient safety margin between the low A4sensing threshold amplitude and the minimum detected A4 amplitude. Theminimum detected A4 amplitude threshold may be set to a multiple of thecurrent low A4 sensing threshold amplitude value, e.g., 1.1, 1.2, 1.25,1.5, 2.0 or other selected multiple. When the minimum detected A4amplitude threshold is not at least a safety margin greater than thecurrent low A4 sensing threshold amplitude, the likelihood of A4 eventundersensing may be increased.

A combination of two or more criteria, including any of those describedabove, may be checked by control circuit 206 at block 534 to determineif conditions for detecting evidence of A4 event undersensing are met.In an illustrative example, control circuit 206 detects evidence of A4event undersensing at block 534 when the applicable cycle count is atleast 5 out of 8 ventricular cycles, the detected A4 count is less thantwo fewer than the applicable cycle count, and the minimum detected A4amplitude is less than or equal to 1.5 times the current low A4 sensingthreshold amplitude setting or the normal A4 count is zero. When thesecriteria are met, control circuit 206 may determine that A4 eventundersensing is likely and the low A4 sensing threshold amplitude shouldbe decreased. Control circuit 206 may advance to block 536 to determineif decrement criteria are met.

By detecting evidence of A4 event undersensing at block 534, controlcircuit 206 may adjust the low A4 threshold amplitude when the low A4threshold amplitude is set too high for reliably sensing A4 events.However, in some instances, the low A4 threshold amplitude may be setappropriately, but true A4 events may not be sensed because A3 events,occurring earlier in the ventricular cycle, are being oversensed,precluding sensing of the later true A4 events. When the cause of A4events being undersensed is only due to the low A4 threshold amplitudebeing set too high, few or no A4 events are expected to be detectedduring the A3 window. However, when the A3 window ending time is set tooearly or the high A4 threshold amplitude is set too low, oversensing ofA3 events may contribute to the undersensing of true A4 events duringthe A4 window. In this case, adjustment to the high A4 thresholdamplitude and/or the A3 window ending time may restore reliable A4sensing, without requiring an adjustment to the low A4 thresholdamplitude.

Accordingly, in some examples, in determining that criteria fordetecting A4 event undersensing during the A4 window are met at block534, control circuit 206 may apply criteria for verifying thatoversensing of the A3 event is not occurring over the N ventricularcycles and contributing to the undersensing of the later A4 event. Insome instances, e.g., with changing heart rate or other factors, the A3window ending time may be set to end near or slightly earlier than theA3 event. In this case, the A3 window ending time may require adjustmentbefore the low A4 threshold amplitude is adjusted. Criteria applied atblock 534 may include comparing the A3 window detection count(determined at block 518 of FIG. 9 ) to a threshold. In some examples,control circuit 206 may require that zero A4 events are detected duringthe A3 window (A3 window detection count is zero over the N ventricularcycles) in order for the A4 event undersensing criteria to be met atblock 534, leading to a possible decrement of the low A4 thresholdamplitude at block 538.

Another condition that may be applied at block 534 for verifying that A3event oversensing is not contributing to undersensing of A4 events mayrequire that relatively few A4 events are sensed early in the A4 window,e.g., within 50 ms or other specified time interval of the A3 windowending time. Several early A4 sensed event signals may occur over the Nventricular cycles when the A3 event signal is occurring after the A3window ending time, resulting in A3 events being oversensed as early A4events. If this is occurring, the low A4 sensing threshold amplitudeshould not be decremented or A3 event oversensing may occur even morefrequently. Instead, the A3 window ending time may require adjusting.Examples of criteria applied by control circuit 206 for verifying thatA3 oversensing is not occurring may include requiring that the early A4count (determined at block 512 of FIG. 9 ) is less than a thresholdnumber, e.g., less than 2 out of the eight ventricular cycles and/orthat the early A4 count is less than the normal A4 count.

When all criteria for determining A4 undersensing during the A4 windoware met at block 534, which may include criteria for verifying that A3oversensing is not occurring, control circuit 206 may determine ifdecrement criteria are met at block 536. The low A4 sensing thresholdamplitude may be decremented when the decrement does not cause the lowA4 sensing threshold amplitude to be less than a minimum value. Othercriteria relating to the minimum detected A4 amplitude and/or currentsetting of the low A4 sensing threshold amplitude may be applied toavoid decreasing the low A4 sensing threshold amplitude when the currentlow A4 sensing threshold amplitude is near the minimum available settingand/or near the minimum detected A4 amplitude.

When the decrement criteria are met at block 536, control circuit 206sets the decrement at block 538 that is used to adjust the low A4sensing threshold amplitude at block 522 of FIG. 9 . The decrement maybe a fixed, predetermined value, e.g., one or more ADC units, e.g., 0.3ms/s². In other examples, the decrement may be an adjustable or scaleddecrement. Control circuit 206 may determine the decrement based on thecurrent setting of the low A4 sensing threshold amplitude and/or theminimum detected A4 amplitude.

When the decrement criteria are not met at block 536, control circuit206 may determine that the low A4 sensing threshold amplitude should notbe decreased from its current value even though evidence of A4 eventundersensing was detected at block 534 with no evidence of A3 eventoversensing. Control circuit 206 may advance to block 546 so that noadjustment is made at block 522 of FIG. 9 .

If the criteria for determining A4 event undersensing during the A4window are not met at block 534, control circuit 206 may advance toblock 540 to determine if regular A4 sensing with high A4 eventamplitude criteria are met. When A4 events having a high amplitude areregularly sensed, an increase in the low A4 sensing threshold amplitudemay be justified. High A4 event amplitude criteria may be met whencontrol circuit 206 determines that regular A4 event sensing during theA4 windows over the N ventricular cycles and/or the sensed A4 eventamplitudes are relatively high compared to the current low A4 sensingthreshold amplitude. For example, when a majority of the N ventricularcycles include A4 events sensed during the A4 window, A4 event sensingmay be determined to be regular. When the peak amplitude of each of theregularly sensed A4 events is greater than a predetermined margin abovethe current setting of the low A4 sensing threshold amplitude, the highA4 amplitude criteria may be determined to be met by control circuit 206at block 540.

In one example, control circuit 206 may determine that the regular A4sensing with high A4 amplitude criteria are met by determining that theapplicable cycle count is greater than a threshold, determining that thedetected A4 count equals the applicable cycle count, determining thatthe normal A4 event count is at least a threshold number and that theminimum detected A4 event amplitude is greater than an adjustmentthreshold. The applicable cycle count threshold may be greater than halfof the N ventricular cycles in some examples. For instance, theapplicable cycle count may be required to be at least five out of theeight ventricular cycles analyzed. When the detected A4 count equals theapplicable cycle count, A4 events are sensed in every A4 window and aredetermined to be regularly sensed. A threshold number of the detected A4events may be required to be normal A4 events (i.e., sensed at least apredetermined time interval after the A3 window ending time). Forexample, the normal A4 cycle count may be required to be greater thanone. The adjustment threshold applied to the minimum detected A4 eventamplitude may be set to the current low A4 sensing threshold amplitudemultiplied by an adjustment factor. For example, the adjustment factormay be 1.25, 1.5, 2.0 or other factor that provides a desired safetymargin between the minimum detected A4 amplitude and the low A4 sensingthreshold. When the minimum detected A4 amplitude as determined at block511 is greater than 1.5 times the current low A4 sensing thresholdamplitude setting, for example, the low A4 sensing threshold amplitudemay be increased.

When control circuit 206 determines that A4 event sensing is occurringduring A4 windows regularly and reliably (e.g., for at least 50% or moreof the N ventricular cycles) and the minimum detected A4 amplitude isgreater than a multiple of the low A4 sensing threshold amplitude,control circuit 206 determines that the criteria are met at block 540and advances to block 542. Control circuit determines whether low A4threshold amplitude increment criteria are met at block 542. Forexample, the current setting of the low A4 sensing threshold amplitudemay be required to be less than a maximum upper limit by a thresholdamount (e.g., by at least one ADC unit or more). Other incrementcriteria may be applied based on the minimum detected A4 event amplitudeand/or the current setting of the low A4 sensing threshold amplitude toavoid introducing A4 event undersensing due to incrementing the low A4sensing threshold amplitude.

When the increment criteria are met at block 542, control circuit 206may advance to block 544 to determine the adjustment increment to beapplied at block 522 of FIG. 9 . The adjustment increment may be set toa fixed predetermined value corresponding to 0.1 m/s², 0.2 m/s², 0.25m/s², 0.3 m/s² or other selected value. As described above, theadjustment increment may be adjustable or scaled based on the minimumdetected A4 amplitude, the current setting of the low A4 sensingthreshold amplitude, the difference between the current setting of thelow A4 sensing threshold amplitude and the maximum or minimum limit, thedifference between the minimum detected A4 amplitude and maximum orminimum limit, the detected A4 count, the normal A4 count, the early A4count, and/or other A4 sensing control parameter metric or anycombination thereof.

When control circuit 206 determines that the criteria for detectingregular A4 event sensing with high A4 amplitude are unmet (block 540) orthat increment criteria are unmet (block 542), control circuit 206 mayset the adjustment to zero at block 546 so that no adjustment to the lowA4 sensing threshold amplitude is made at block 522 of FIG. 9 .

It is to be understood that, when incrementing the low A4 sensingthreshold amplitude by a predetermined fixed increment that would causethe low A4 sensing threshold amplitude to exceed a maximum allowablesetting, control circuit 206 may set the adjustment increment to aportion of the fixed increment so that the low A4 sensing thresholdamplitude is adjusted to the maximum limit without exceeding it.Likewise, in some instances, the adjustment decrement set at block 538may be a portion of a predetermined fixed decrement when an adjustmentby the fixed decrement would cause the low A4 sensing thresholdamplitude to be less than a minimum limit. For example, the minimumsetting may be an acceleration of 0.8 m/s². When the predetermined,fixed decrement is 0.3 m/s² and the current value of the low A4 sensingthreshold amplitude is 0.9 m/s², decreasing the A4 sensing thresholdamplitude by the fixed decrement would cause it to go below the minimumlimit. In this case, the low A4 sensing threshold amplitude may bedecremented to the minimum allowable setting of 0.8 m/s² instead of byone whole, predetermined fixed decrement value.

The various criteria and conditions described in conjunction with theflow chart 420 of FIG. 10 are described as being applied to the A4sensing control parameter metrics in a particular order. It is to beunderstood, however, that the criteria may be applied in a differentorder than the order shown here. For example, one or more incrementcriteria of block 532 may be applied before and/or after determining ifearly A4 window sensing criteria are met or vice versa. Likewise, one ormore decrement criteria may be applied before and/or after determiningif A4 undersensing criteria are met at block 534 or vice versa. Forexample, control circuit 206 may check if the low A4 sensing thresholdamplitude is already at a maximum or minimum limit before checkingconditions that would cause an increment or a decrement, respectfully.Furthermore, it is contemplated that control circuit 206 may beconfigured to evaluate multiple criteria sequentially, simultaneously orin parallel combinations to arrive at a decision to increment, decrementor not adjust the low A4 sensing threshold amplitude.

When blocks 530, 534 and 540 are performed in the order show, controlcircuit 206 may give priority to checking for A3 event oversensing bychecking for early A4 window sensing conditions being met at block 530.A3 event oversensing leads to a loss of atrial tracking by theventricular pacing pulses during a VDD pacing mode. Ventricular pacingpulses that are tracking the A3 event, which is a ventricular event, maybe delivered at relatively short ventricular cycles leading to fastpacing rates. As such, a first priority may be to ensure that A3 eventoversensing is not occurring starting with the criteria described inconjunction with block 530.

The second priority may be to detect A4 event undersensing when A3 eventoversensing is not occurring, as determined when criteria are met atblock 534. In this case, ventricular pacing pulses may be deliveredasynchronously with the A4 event due to VV pacing intervals expiringwithout A4 sensed event signals being generated to trigger an AV pacinginterval. As such, while ventricular rate support is being provided,atrioventricular synchrony may be lost during A4 event undersensing.Accordingly, detecting undersensing of A4 events within the A4 windowwhen no A3 event oversensing is occurring may be the second prioritycriteria used by control circuit 206 in making a decision to adjust thelow A4 sensing threshold amplitude.

The lowest priority conditions for adjusting the low A4 sensingthreshold amplitude may be applied at block 540 for increasing the lowA4 sensing threshold amplitude in response to regular sensing of highamplitude A4 events. In this case, ventricular pacing may be properlytracking A4 events with reliable A4 sensed event signals. An adjustmentto the low A4 sensing threshold amplitude may be optional under theseconditions, but an increment may promote continued, reliable A4 eventsensing while reducing any likelihood of oversensing A3 events or othermotion signals occurring in the A4 window. As indicated above, however,while blocks 530, 534 and 540 are shown in a particular order that maycorrespond to a prioritized hierarchy based on how the A3 oversensing,A4 undersensing, and reliable A4 sensing may affect ventricular trackingof the atrial events, it is recognized that these criteria may beapplied in parallel or simultaneous operations to arrive at anadjustment decision. Once an adjustment decision is made at block 538,544 or block 546, control circuit 206 may advance to block 522 of theflow chart 500 of FIG. 9 to perform the adjustment of the low A4 sensingthreshold amplitude accordingly.

FIG. 11 is a flow chart 600 of an illustrative method for determiningwhen low A4 sensing threshold adjustment criteria are met and settingthe adjustment increment or decrement according to one example. Controlcircuit 206 may execute instructions stored in memory 210 to perform thevarious comparisons and decisions attributed to control circuit 206 inconjunction with FIG. 11 and other flow charts presented herein. Thefunctionality may be implemented in any combination of hardware,firmware and/or software. The process of flow chart 600 may be performedafter every N ventricular cycles using the A4 sensing control parametermetrics determined from the N ventricular cycles as described above inconjunction with FIG. 9 . The A4 sensing control parameter metrics mayinclude counts of all sensed A4 events, early A4 events, normal A4events, A4 events sensed during the A3 window, and the minimum detectedA4 event amplitude, as examples. The process of flow chart 600 isperformed by control circuit 206 while control circuit 206 operates inan atrial tracking ventricular pacing mode (e.g., VDD pacing mode).

At block 602, control circuit 206 compares the early A4 count to athreshold value and to the normal A4 count. Control circuit 206 maydetect A3 event oversensing in response to the early A4 event countbeing greater than a threshold, e.g., greater than a predeterminedpercentage of N (the number of ventricular cycles) and greater than thenormal A4 event count. When the early A4 count is greater than thethreshold value (e.g., 2 or more) and greater than or equal to thenormal A4 count, control circuit 206 compares the current low A4 sensingthreshold amplitude to an increment threshold at block 604. When the lowA4 sensing threshold amplitude is less than or equal to the incrementthreshold, control circuit 206 increases the low A4 sensing thresholdamplitude at block 606, which may be by one increment, e.g., by 0.3m/s², to no more than an upper maximum limit, e.g., 5.0 m/s². When thelow A4 sensing threshold amplitude is already greater than a multiple,e.g., twice, the minimum setting of the low A4 sensing thresholdamplitude, the low A4 sensing threshold amplitude may not be increasedby control circuit 206. In one example, the low A4 sensing thresholdamplitude is not incremented when it is more than 1.6 m/s², twice theminimum setting of 0.8 m/s².

Blocks 602 and 604 generally correspond to criteria applied fordetermining that early A4 sensing conditions are met as evidence ofpossible A3 event oversensing and an increment criterion is met,respectively. By increasing the low A4 sensing threshold amplitude inresponse to detected frequent early A4 sensed events, the likelihood ofpersistent A3 event oversensing may be reduced. As such, the criteriaapplied at blocks 602 and 604 reflect a tradeoff between protectingagainst oversensing of the A3 event (by increasing the low A4 sensingthreshold) while avoiding over-increasing the low A4 sensing thresholdamplitude which may cause intermittent or persistent A4 eventundersensing. It is desirable to increase the low A4 sensing thresholdamplitude enough to avoid oversensing of the A3 event or other motionsignals while keeping it low enough to reliably sense A4 events duringthe A4 window, particularly when the heart rate is relatively highand/or the A4 event signal has a relatively low amplitude.

When either of the criteria at block 602 or 604 are unmet, controlcircuit 206 advances to block 608 to compare the applicable cycle countto a threshold count, e.g., 5 when N=8 ventricular cycles that areevaluated for determining the A4 sensing control parameter metrics. Whencontrol circuit 206 determines that at least a threshold number ofventricular cycles are applicable cycles (cycles with an A4 windowstarted), and the detected A4 count is less than the applicable cyclecount minus X at block 610 (e.g., the detected A4 count is less than theapplicable cycle count minus 2), A4 event undersensing criteria may bemet. In other examples, the detected A4 count as a percentage of theapplicable cycle count may be compared to a threshold percentage, e.g.,80%, 70% or other percentage. When the detected A4 count is less than apercentage threshold of the applicable cycle count, A4 undersensing maybe suspected, and decreasing the low A4 sensing threshold amplitude maybe warranted.

Control circuit 206 may determine if A3 event oversensing is likely atblock 612 by comparing the early A4 count to a threshold count and/orcomparing the early A4 count to the normal A4 count. When the early A4count is less than 2 or another selected threshold, or the early A4count is less than the normal A4 count, A3 oversensing is unlikely,supporting a determination of A4 undersensing during the A4 window and aneed to decrement the low A4 sensing threshold amplitude.

At block 612, control circuit 206 may additionally or alternativelydetermine if the A3 window detection count (number of sensed A4 eventsthat occur during the A3 window) is less than a threshold, e.g., lessthan 1 or a zero A3 window detection count. When zero A4 events aresensed during the A3 window and the early A4 count is less than athreshold count and less than the normal A4 count, A3 oversensingconditions are unmet, supporting a determination that criteria fordetecting A4 undersensing are met and warranting a decreased low A4sensing threshold amplitude.

Before decreasing the low A4 sensing threshold amplitude, controlcircuit 206 may determine whether the minimum detected A4 amplitude isless than a threshold amplitude at block 614. For example, controlcircuit 206 may determine that the minimum detected A4 amplitude is lessthan or equal to the current low A4 sensing threshold amplitudemultiplied by a safety factor, e.g., multiplied by 1.5. In someexamples, control circuit 206 may determine that the normal A4 count iszero at block 614, justifying decrementing the low A4 sensing thresholdamplitude even when the minimum detected A4 amplitude is not greaterthan the current low A4 sensing threshold amplitude multiplied by asafety factor.

The criteria applied at blocks 608, 610 and 612 generally correspond tocriteria applied at block 534 of FIG. 10 for determining A4 undersensingwithout A3 oversensing. When the criteria at blocks 608, 610 and 612 aremet and the decrement criteria are met at block 614, control circuit 206may decrease the low A4 sensing threshold amplitude at block 616 by oneADC unit or other selected decrement, but no less than the minimumallowable low A4 sensing threshold amplitude, e.g., 0.8 m/s².

Control circuit 206 may apply the A3 oversensing criteria of block 612and the decrement criteria at block 614 to avoid decreasing the low A4sensing threshold amplitude when A4 sensing and atrial tracking of theventricular pacing pulses may be temporarily lost. The low A4 sensingthreshold amplitude may be decreased only when the detected A4 count isless than the applicable cycle count by at least 2 (or other selecteddifference threshold) and the minimum detected A4 amplitude is low (lessthan a threshold) or a normal detected A4 count is zero, all of whichcriteria support a detection of A4 event undersensing associated withlow amplitude A4 event signals. When the A3 window detection count ishigher than a threshold, or the early A4 count is higher than athreshold (e.g., higher than the normal A4 count), A3 oversensing may beoccurring, which may be causing the relatively later A4 event signal togo unsensed in some ventricular cycles. However, decreasing the low A4sensing threshold amplitude may be unwarranted if a transient conditioncauses temporary A3 event oversensing associated with a temporary lossof A4 sensing during the current N ventricular cycles. Control circuit206 may be configured to avoid decrementing the low A4 sensing thresholdunder such conditions, e.g., by checking the amplitude of the A4 eventsignals using the minimum detected A4 amplitude or another A4 signalamplitude metric at block 614.

A temporary loss of A4 sensing may occur when premature ventricularcontractions (PVCs) occur during the N ventricular cycles, the patientmakes a posture change or other conditions. In an illustrative example,if two non-consecutive premature ventricular contractions (PVCs) occurin the 8 ventricular cycles, the applicable cycle count may reach six.A4 windows may not be started during the two PVC cycles when the PVCoccurs during the A3 window. The A4 event may not be sensed on the nextventricular cycle following each PVC because the A4 window may notcorrespond to the atrial rate on the first cycle after a PVC. If the A4event is missed on just one other cycle, the number of detected A4events out of the eight ventricular cycles may be only three. In thisscenario, the A4 event undersensing criteria of the applicable cyclecount being at least 5 (six applicable cycles) and the detected A4 countbeing less than two fewer than the applicable cycle count (only threesensed A4 events) are satisfied. However, by applying the minimumdetected A4 amplitude criteria at block 614, the decrement criteria maybe unmet. Of the three A4 event signals that are sensed, the minimumdetected A4 amplitude may be greater than a safety factor times the lowA4 sensing threshold amplitude, indicating that the low A4 sensingthreshold amplitude is at an appropriate value but other factors mayhave caused temporary, transient A4 undersensing criteria to be met, inthis case two PVCs.

Thus, this check of the minimum detected A4 amplitude when A4undersensing criteria are met avoids decreasing the low A4 sensingthreshold amplitude when the A4 event signals have a relatively highamplitude but other factors, e.g., PVCs, may be the underlying cause ofthe A4 undersensing criteria being met during the N ventricular cycles.Holding the low A4 sensing threshold amplitude at the current value mayallow regular A4 sensing to be regained during the next N ventricularcycles, with AV synchrony being restored by the atrial trackingventricular pacing, without requiring an adjustment to the low A4sensing threshold amplitude. In this example, application of the minimumdetected A4 amplitude criteria (block 614) avoids decreasing the low A4sensing threshold amplitude even when the detected A4 count is low (atblock 610), e.g., due to a temporary loss in sensing the A4 events dueto PVCs or other transient, non-sustained changes in the heart rhythm,patient position or activity, or other factors.

When any of the criteria at blocks 608, 610, 612 or 614 are unmet,control circuit 206 advances to block 618 to determine if regular A4sensing is occurring with relatively high A4 event signal amplitudes. Atblocks 618 and 620, control circuit 206 determines if regular A4 eventsensing is occurring by comparing the detected A4 count to theapplicable cycle count and comparing the normal A4 count to a thresholdcount, respectively. In one example, when the detected A4 count equalsthe applicable cycle count (A4 sensed event signal generated in every A4window that occurs over N ventricular cycles) and the normal A4 count isat least 1, control circuit 206 determines that regular A4 sensing isoccurring. The criteria applied at blocks 618 and 620 correspondgenerally to criteria that may be applied at block 540 of FIG. 10 fordetermining regular A4 sensing with high A4 amplitude. When thesecriteria are met, control circuit 206 advances to block 622 to determinewhether increment criteria are met.

At block 622, control circuit 206 may compare the minimum detected A4amplitude to a threshold. The threshold may be based on the currentsetting of the low A4 sensing threshold amplitude multiplied by a safetyfactor, e.g., 1.5. When the minimum detected A4 amplitude is greaterthan the threshold at block 622, control circuit 206 determines that thelow A4 sensing threshold amplitude may be safely increased at block 606without impairing or disrupting the regular A4 event sensing. Controlcircuit 206 may increase the low A4 sensing threshold amplitude by apredetermined increment, e.g., 0.3 m/s² or other selected increment.Other multiples of the low A4 sensing threshold amplitude may be used asa threshold for comparing to the minimum detected A4 amplitude asdescribed above in conjunction with FIG. 7 . The multiple used to setthe increment criteria at block 622 effectively sets the minimum safetymargin required between the low A4 sensing threshold amplitude and theminimum detected A4 amplitude to promote reliable A4 event sensing.

In some instances, control circuit 206 may determine that none of thecriteria applied at blocks 602, 604, 608, 610, 612, 614, 618, 620, and622 are met such that no adjustment is performed at block 624. In thissituation, A4 events may be sensed regularly, and the low A4 sensingthreshold amplitude is deemed appropriate based on the minimum detectedA4 amplitude being at least an acceptable safety margin of the currentlow A4 sensing threshold amplitude.

At other times, however, A4 events may be undersensed due to A3 eventoversensing during the A3 window or after the A3 window ending time yetincrement or decrement criteria may not be met. In this case, adjustmentof the low A4 sensing threshold amplitude may not correct the situationof A3 event oversensing. Rather an adjustment to the high A4 sensingthreshold amplitude applied during the A3 window and/or adjustment ofthe A3 window ending time may be a more appropriate response to restorereliable A4 event sensing. Techniques for automatically adjusting thehigh A4 sensing threshold amplitude and the A3 window ending time aredescribed below.

FIG. 12 is a diagram 700 of an EGM signal 702 that may be produced bysensing circuit 204 and a motion signal 710 produced by motion sensor212, shown here as an acceleration signal (ACC). Diagram 700 depicts A4sensing control parameters and A4 sensing control parameter metrics thatmay be determined from motion signal 710 by control circuit 206 for usein adjusting at least some of the A4 sensing control parameters. The A4sensing control parameters depicted in FIG. 12 include the A3 windowending time 744, the high A4 sensing threshold amplitude 720 applied tomotion signal 710 during the A3 window 740, and the low A4 sensingthreshold amplitude 722 applied after the A3 window ending time 744during the A4 window 750. The A4 sensing control parameters furtherinclude the post-ventricular atrial blanking period 730 and thepost-ventricular atrial refractory period 732. A4 sensing controlparameter metrics that may be determined from motion signal 710 bycontrol circuit 206 include an A3 event time 726, A3 event amplitude760, an A4 event time 728 and an A4 event amplitude 762. Each of theseA4 sensing control parameter metrics may be determined for use bycontrol circuit 206 in adjusting the A3 window ending time 744 and/orthe high A4 sensing threshold amplitude 720.

The EGM signal 702 includes an R-wave 704, which may be an intrinsicR-wave sensed by sensing circuit 204. Sensing circuit 204 may generateand R-wave sensed event signal 706 which starts a post-ventricularatrial blanking period 730 (during which no A4 events are sensed) and apost-ventricular atrial refractory period (PVARP) 732, during which A4events may be sensed after expiration 742 of blanking period 730 but donot cause an AV pacing interval to be started by pace timing circuit 242(FIG. 3 ). The PVARP 732 may be adjusted every N ventricular cycles insome examples, based on a ventricular rate during the N ventricularcycles. Post-ventricular atrial blanking period 730 may extend up to 550ms in some examples. PVARP 732 may be set to at least the blankingperiod 730 and to no longer than the A3 window ending time 744 but willgenerally expire earlier than the A3 window ending time 744. Forexample, the A3 window 740 may start at 550 ms (end of blanking period730) and extend to an ending time 744 that is 600 ms up to 1000 ms afterthe ventricular event 706. PVARP 732 may expire at 550 to 600 ms afterthe R-wave sensed event signal 706.

The PVARP 732 may be adjusted based on a ventricular rate metricdetermined as the median or nth longest ventricular cycle interval outof N ventricular cycles e.g., the 4^(th) longest out of eightventricular cycles. The ventricular cycle intervals, e.g., RR intervals,may each start and end with a sensed R-wave or ventricular pacing pulse.In one example, control circuit 206 sets PVARP 732 to the ventricularrate metric less an offset, e.g., a median ventricular cycle intervalless 100 to 200 ms. In one example, PVARP 732 is adjusted after every N(e.g., 8) ventricular cycles to the nth longest (e.g., fourth longest)ventricular cycle interval minus 130 ms, but not less than a minimumPVARP (e.g., 500 ms) and not more than a maximum PVARP (e.g., 600 ms).The offset used to set the PVARP 732 based on the ventricular ratemetric may include a PVARP adjustment offset, e.g., 100 ms, plus the AVpacing interval, e.g., 10 ms, 20 ms, or 30 ms as examples. In this way,the PVARP 732 may be shortened during fast heart rates but is notextended longer than a maximum, e.g., 600 ms, during slow heart rates topromote non-refractory sensing of the A4 event signals.

During the blanking period 730, the A1 event 712 and A2 event 714 occurand are not sensed by atrial event detector circuit 240. At theexpiration of the blanking period 730, control circuit 206 starts the A3window 740 which extends to the A3 window ending time 744. The A3 event716 occurs during the A3 window 740. In the example shown, the A3 event716 is not sensed as an A4 event since the motion signal 710 does notcross the high A4 sensing threshold amplitude 720 applied during A3window 740. However, control circuit 206 may set a low A3 thresholdamplitude 724 that is applied during the A3 window 740 for use indetermining the time of the A3 event 716 as an A4 sensing controlparameter metric used in adjusting the A3 window ending time 744. Thelow A3 threshold amplitude 724, which is also referred to herein as “aventricular diastolic event threshold amplitude,” may be set equal tothe low A4 sensing threshold amplitude 722 or lower, e.g., 75% of thelow A4 sensing threshold amplitude. The A3 event 716 crosses the low A3threshold amplitude 724 at a time 726 from the R-wave sensed eventsignal 706 (the start of the ventricular cycle). This low A3 sensingthreshold crossing time 726 may be referred to as the “A3 event time”726. The low A3 sensing threshold crossing time 726 is shown as thelatest, negative-going crossing time of the A3 sensing threshold 724. Inother examples, a positive-going crossing time of the A3 sensingthreshold 724 may be determined as the A3 event time.

During some ventricular cycles, the motion signal 710 may cross the lowA3 threshold amplitude 724 more than once during the A3 window 740. Insome examples, therefore, the A3 event time 726 may be determined as thelatest crossing time of the low A3 threshold amplitude 724 during A3window 740, which may be the latest negative-going crossing of the lowA3 threshold amplitude 724. Control circuit 206 may adjust the A3 windowending time 744 based on the latest crossing time of the low A3threshold amplitude 724 to avoid setting the A3 window ending time 744too early, which may lead to oversensing of the A3 event 716 during theA4 window 750. As further described below, control circuit 206 maydetermine the A3 event time 726 during each of N ventricular cycles foruse in adjusting the A3 window ending time 744 after every N ventricularcycles or less often, as needed. Control circuit 206 may therefore beconfigured to determine a latest time in the A3 window 740 that themotion signal 710 is greater than the low A3 threshold amplitude 724 sothat the A3 window ending time 744 may be set appropriately after the A3event signal 716 has ended (or at least returned to a low amplitude thatwill not be oversensed as an A4 event).

In other ventricular cycles, the motion sensor signal 710 may not crossthe low A3 threshold amplitude 724. In this case, the A3 event time maynot be determined. Control circuit 206 may count the number ofventricular cycles that the A3 event time is not determined due to nolow A3 sensing threshold crossing. In other examples, the A3 event timemay be set to a minimum predetermined or default value, e.g., 550 ms, toindicate that the A3 window ending time 744 may be adjusted to arelatively early time after the ventricular event 706 without concern ofthe A3 event signal 716 having an amplitude that is greater than the lowA4 sensing threshold amplitude 722. In still other ventricular cycles, apremature ventricular contraction or other fast ventricular beat mayoccur before the A3 window 740 or before the A3 window ending time 744,causing the A3 window 740 to not be started or at least not be completedduring the ventricular cycle. The A3 event time may not be determined bycontrol circuit 206 for the ventricular cycle when the A3 window 740 isnot started or does not reach the A3 window ending time 744 in someexamples. Control circuit 206 may count the number of ventricular cyclesthat the A3 event time is not determined due to no or incomplete A3windows.

The low A3 threshold amplitude 724 applied during the A3 window 740 fordetecting the A3 event time 726 may be set based on the low A4 sensingthreshold amplitude 722 in some examples. For instance, the low A3threshold amplitude 724 may be set to 60%, 70%, 75%, 80%, 90%, 100% orother selected percentage of the low A4 sensing threshold amplitude 722that is applied after the A3 window ending time 744, during the A4window 750. In one example, low A3 threshold amplitude 724 is set to be75% of the low A4 sensing threshold amplitude 722 and may be adjusted upor down every N ventricular cycles with the low A4 sensing thresholdamplitude 722 as it is adjusted according to any of the examplesdescribed above in conjunction with FIGS. 9-11 .

Control circuit 206 may be configured to determine the A3 eventamplitude 760 during each of the N ventricular cycles. Control circuit206 may determine the A3 event amplitude 760 for use in adjusting thehigh A4 sensing threshold amplitude 720 as further described below. TheA3 event amplitude 760 may be determined as the maximum peak amplitudeof the motion signal 710 during the A3 window 740. In some instances,the A3 event amplitude 760 is determined as the maximum sample pointamplitude that occurs after the low A3 threshold amplitude 724 iscrossed by the motion sensor signal 710. However, the maximum peakamplitude 760 does not necessarily always occur after the latest motionsensor signal crossing time of the low A3 threshold amplitude,particularly when the latest negative-going crossing time is determinedas the A3 event time 726. As such, the A3 event amplitude 760 may occurearlier or later in the A3 window 740 than the A3 event time 726. Thetime of the A3 event amplitude 760 may be independent of the A3 eventtime 726. Though, in some examples, the A3 event amplitude 760 may bedetermined as the maximum amplitude that is greater than or equal to thelow A3 threshold amplitude 724 such that, in some instances, the A3event amplitude 760 and the A3 event time 726 could occur at the samesample point.

During some ventricular cycles, the motion signal 710 may not cross thelow A3 threshold amplitude 724 such that an A3 event time is notdetermined. When the low A3 threshold amplitude 724 is not crossed, theA3 event amplitude 760 may or may not be determined as the maximumsample point amplitude during the A3 window 740. When the low A3threshold amplitude 724 is not crossed, control circuit 206 maydetermine the A3 event amplitude to be unknown. As such, in someexamples, control circuit 206 determines the A3 event amplitude as themaximum amplitude that is greater than (or equal to) the low A3threshold amplitude 724.

In addition to determining the A3 event amplitude 760 and the A3 eventtime 726, control circuit 206 may be configured to determine the A4event amplitude 762 and the A4 event time 728 during the A4 window 750of each of the ventricular cycles (of the N ventricle cycles) that an A4window 750 is started. The A4 window 750 is started when the A3 window740 ends without a sensed A4 event or a sensed R-wave. The A4 event time728 may be determined as the time interval from the ventricularelectrical event 706 that starts the ventricular cycle (in this cyclethe R-wave sensed event signal 706 but in other cycles a ventricularpacing pulse) until the maximum amplitude 762 of the motion signal 710during the A4 window 750 (after the A3 window ending time 744). In otherexamples, control circuit 206 may determine the A4 event time 728 as thetime interval from the most recent ventricular electrical event to theearliest crossing time of the low A4 sensing threshold amplitude 722.Control circuit 206 may determine the A4 event time 728 for use inadjusting the A3 window ending time 744 as described below, alone or incombination with the A3 event time 726.

In some ventricular cycles, the motion signal 710 may not cross the lowA4 sensing threshold amplitude 722 even though an A4 window 750 isstarted. The low A4 sensing threshold amplitude 722 may be set too high,a ventricular pace or sensed R-wave may occur before the low A4 sensingthreshold amplitude 722 is crossed, or an atrial beat may not occurduring the A4 window to cause the A4 event 718. As such, the A4 eventtime may not be determined by control circuit 206 for every ventricularcycle that includes an A4 window. Control circuit 206 may or may notdetermine the A4 event amplitude 762 when the A4 event time is notdetermined for a given ventricular cycle. Control circuit 206 maydetermine the A4 event amplitude 762 as the maximum sample pointamplitude during the A4 window 750 that is greater than the low A4sensing threshold amplitude 722.

As shown in FIG. 12 , when the A4 event 718 is sensed by atrial eventdetector circuit 240 (FIG. 3 ) outside the PVARP 732, e.g., in responseto the motion signal 710 crossing the low A4 sensing threshold amplitude722, an AV pacing interval 764 is started by pace timing circuit 242.Pulse generator 202 generates a ventricular pacing pulse 708 in responseto the expiration of the AV pacing interval 764, which ends theventricular cycle and the A4 window 750. During other ventricularcycles, atrial event detector circuit 240 may sense an A4 event inresponse to the motion signal 710 crossing the high A4 sensing thresholdamplitude 720 during the A3 window 740, e.g., when the heart rate ishigh and the A4 event signal is fused with the A3 event signal resultingin a combined high amplitude fused A3/A4 event signal.

If the A4 event is sensed during the A3 window 740 outside of the PVARP732, the AV pacing interval 764 is started by pace timing circuit 242.Pulse generator 202 generates a pacing pulse upon expiration of the AVpacing interval 764 to provide atrial tracking of A4 event sensed afterPVARP 732, during either A3 window 740 or A4 window 750.

It is noted that when the atrial event detector circuit 240 senses theA4 event during the A3 window 740, the A3 window 740 may be extendeduntil the next ventricular event. For example, pace timing circuit 242may start the AV pacing interval 764 in response to the A4 event beingsensed during the A3 window 740. The AV pacing interval 764 may extendbeyond the scheduled A3 window ending time 744. However, to avoidanother A4 sensing threshold crossing during the ventricular cycle,control circuit 206 may extend the A3 window 740 until the AV pacinginterval expires and the ventricular pacing pulse is delivered. Controlcircuit 206 may extend the A3 window 740 until the next ventricularevent, sensed or paced, when the high A4 sensing threshold amplitude 720is crossed so that the A4 window 750 is not started during the sameventricular cycle. The A3 window is extended to avoid another A4 sensingthreshold amplitude crossing in the same ventricular cycle. In thiscase, the A4 event time 728 and the A4 event amplitude 762 are notdetermined since the A4 window 750 is not started. The A3 window 740 maybe effectively extended by holding the high A4 sensing thresholdamplitude 720 until the next ventricular electrical event, withoutdecreasing the A4 sensing threshold to the low A4 sensing thresholdamplitude 722.

FIG. 13 is a flow chart 760 of a method performed by control circuit 206for determining A4 sensing control parameter metrics from a motionsignal and adjusting the A3 window ending time according to one example.At block 762, control circuit 206 determines that the current operatingmode includes atrial event sensing. For example, control circuit 206 maybe operating in an atrial tracking ventricular pacing mode, e.g., atemporary or permanent VDD pacing mode that includes A4 event sensing.At other times, control circuit 206 may be operating in a non-atrialtracking ventricular pacing mode but with atrial sensing (dual chambersensing) enabled, e.g., a VDI pacing mode. During a VDD or VDI pacingmode, adjustments to the A3 window ending time may be enabled. Controlcircuit 206 may disable determining A4 sensing control parameter metricsand adjustments of the A3 window ending time when the pacing mode is arate responsive pacing mode, e.g., VDIR pacing mode, or a single chambersensing mode, e.g., VVI or VVIR pacing mode.

With reference to the A4 sensing control parameter metrics describedabove in conjunction with FIG. 12 , in response to a ventricularelectrical event (sensed R-wave or ventricular pacing pulse) starting anew ventricular cycle, control circuit 206 may determine the A3 eventtime 726 at block 764. Control circuit 206 may determine the A4 eventtime 728 at block 766 for the ventricular cycle when an A4 window 750 isstarted and the low A4 sensing threshold amplitude is crossed. At block768, control circuit 206 determines if N ventricular cycles haveelapsed, e.g., eight ventricular cycles in the examples given above. Ifnot, control circuit 206 returns to block 764 to continue determiningthe A3 event time and A4 event time for each ventricular cycle.

As described above, the A3 event time and/or A4 event time may not bedeterminable for some ventricular cycles, e.g., when an early R-wave issensed so that the A3 and/or A4 window is not started or the motionsignal does not cross the low A3 threshold amplitude or the low A4sensing threshold amplitude. When the motion signal does not cross thelow A3 threshold amplitude during a ventricular cycle, control circuit206 may determine the A3 event time for that cycle as unknown. In otherexamples, the A3 event time may be set according to a specified valuewhen the low A3 threshold amplitude is not crossed. For example, controlcircuit 206 may set the A3 event time to the minimum available settingof the A3 window ending time or the minimum available setting of the A3window less a specified interval, e.g., less 50 ms. In one example, theminimum A3 window ending time is 600 ms so that control circuit 206 setsthe A3 event time to 550 ms when the low A3 threshold amplitude is notcrossed by the motion signal. When the motion signal amplitude staysbelow the low A3 threshold amplitude throughout the A3 window, the A3window may be set to a minimum ending time since the risk of A3 eventoversensing is minimal.

When an A4 window is not started in a ventricular cycle, the A4 eventtime for that ventricular cycle is unknown. Examples of when the A4window 750 is not started are described above, e.g., when a ventricularelectrical event occurs before the A3 window ending time 744 or the A3window 740 is extended due to an A4 sensed event during the A3 window740.

When the A4 window is started but the motion sensor signal does notcross the low A4 sensing threshold amplitude during the A4 window,control circuit 206 may set the A4 event time for that ventricular cycleto a default value. The default value may be based on the programmedlower ventricular pacing rate or a rate smoothing rate. Control circuit206 may set a rate smoothing rate interval based on the actualventricular rate so that the VV pacing interval may be set to a ratesmoothing rate interval, which may be shorter than the lower ventricularpacing rate interval, in order to avoid an abrupt ventricular ratechange to the lower ventricular pacing rate when an A4 event is notsensed. The rate smoothing interval may be adjusted gradually toward thelower rate interval when A4 events are not being sensed. When the motionsensor signal 710 does not cross the low A4 sensing threshold amplitude722 during the A4 window 750, the A4 event time may be set to the ratesmoothing interval plus an offset (which may be zero or other selectedvalue) indicating that the A4 event time is relatively long, after theVV pacing interval. In other examples, the A4 event time may be set tothe lower rate interval. In this way, an A4 event time may be determinedby control circuit 206 when the A4 window is started to be at least theVV pacing interval even when the motion sensor signal 710 does not crossthe low A4 sensing threshold amplitude 722 before the VV pacing intervalexpires.

After N ventricular cycles (“yes” branch of block 768), control circuit206 may determine an A3 time metric at block 770 and an A4 time metricat block 772. The A3 time metric is determined from the individual A3event times determined over the N ventricular cycles, and the A4 timemetric is determined from the individual A4 event times determined overthe N ventricular cycles. In various examples, as further describedbelow, the A3 window ending time may be adjusted based on one or both ofthe A3 time metric and the A4 time metric. For instance, the A3 windowending time may be adjusted to be at least a threshold time intervallater than the A3 time metric, at least a threshold time intervalearlier than the A4 time metric, or at some interval between the A3 timemetric and the A4 time metric.

The A3 time metric may be determined by control circuit 206 as the meanor median of all A3 event times determined from the N ventricularcycles. In other examples, the A3 time metric may be determined as aspecified n^(th) shortest (or longest) A3 event time determined from theN ventricular cycles. For example, when eight ventricular cycles areevaluated, the fourth shortest A3 event time may be determined as the A3time metric at block 770. When less than four A3 event times areobtained from the N ventricular cycles, the A3 time metric may bedetermined as unknown, e.g., due to too few A3 windows being started. Inother examples, the longest A3 event time of any A3 event timesdetermined may be determined as the A3 event time metric.

At block 772, control circuit 206 determines the A4 time metric from theA4 event times determined during the N ventricular cycles. The A4 timemetric may be determined as the mean, median or n^(th) shortest (orlongest) A4 event time. In one example, the A4 time metric is set to thefourth shortest A4 event time determined out of eight ventricularcycles. When fewer than four (or other specified requisite number of) A4event times are determined for the N ventricular cycles, control circuit206 may determine the A4 time metric as being unknown.

At block 774, control circuit 206 determines a target A3 window endingtime based on the A3 time metric and/or the A4 time metric. In oneexample, the target A3 window ending time is determined as the A3 timemetric plus a fraction or percentage of the difference between the A4time metric and the A3 time metric. For example, control circuit 206 maydetermine the difference between the A3 time metric and the A4 timemetric at block 774 then add 20%, 25%, 30%, 33%, 40% or other selectedpercentage of the difference to the A3 time metric to determine thetarget A3 window ending time. In this way, the target A3 window endingtime is set to be a safety margin later than the expected A3 event time.In one example, the target A3 window ending time is the A3 time metricplus one-third of the difference between the A3 time metric and the A4time metric. When either of the A3 time metric or the A4 time metric isdetermined to be unknown, the target A3 window ending time may bedetermined by control circuit 206 as being unknown or held to apreviously determined value.

The percentage or fraction of the difference between the A3 time metricand the A4 time metric used to set the target A3 window ending time maybe adjustable in some examples, e.g., based on the current ventricularrate and or based on the magnitude of the difference. A ventricular ratemetric may be determined over the N ventricular cycles, which is used indetermining the rate smoothing pacing interval in some examples. Thetarget A3 window ending time may be set to the A3 time metric plus afirst percentage of the difference between the A4 time metric and the A3time metric when the ventricular rate metric is less than a ratethreshold. The target A3 window ending time may be set to the A3 timemetric plus a second percentage of the difference (different than thefirst percentage) when the ventricular rate metric is greater than orequal to the rate threshold. For instance, a higher percentage of thedifference (e.g., 30%) may be used during relatively slower ventricularrates (e.g., less than 80, 90 or 100 beats per minute) than thepercentage of the difference (e.g., 15%) during relatively higherventricular rates (e.g., greater than 80, 90 or 100 beats per minute).

In some examples, the method used by control circuit 206 to determinethe target A3 window ending time is selected according to the pacingoperating mode in effect. The target A3 window ending time may bedetermined differently when the pacing mode is VDD than when the pacingmode is VDI for example. During the VDD pacing mode, control circuit 206may determine the target A3 window ending time based on a combination ofthe A3 time metric and the A4 time metric as described above. During theVDI pacing mode, control circuit 206 may determine the target A3 windowending time based on the A3 time metric and not the A4 time metric. Forexample, control circuit 206 may determine the target A3 window endingtime as the A3 time metric plus a specified offset, e.g., plus 50 ms,100 ms, 150 ms or other selected offset. The offset may be scaled oradjusted based on the actual ventricular rate in some examples. Duringthe non-atrial tracking VDI pacing mode, the atrial rate may bedifferent than the ventricular rate such that A4 event signals do notreliably occur during the A4 window. The A3 event times may bedetermined, but any A4 event times may be unreliable. As such, duringthe VDI pacing mode, control circuit 206 may only use the A3 time metricand not the A4 time metric in determining the target A3 window endingtime at block 774.

At block 776, control circuit 206 determines if A3 window ending timeadjustment criteria are met. In order to determine if adjustmentcriteria are met, control circuit 206 may be configured to determine ifthe target A3 ending time is known or unknown and for what reason (e.g.,too few A3 windows during the N ventricular cycles or too few low A3threshold amplitude crossings during the A3 windows over the Nventricular cycles). Control circuit 206 may additionally oralternatively determine whether the A4 time metric is known or unknown.Control circuit 206 may be configured to compare the A3 time metric tothe A4 time metric or compare their difference to a difference thresholdin order to determine if adjustment criteria are met at block 776.Additionally or alternatively, control circuit 206 may be configured tocompare the current value of the A3 window ending time 744 to the targetA3 window ending time at block 776 to determine if adjustment criteriaare met. Examples of methods control circuit 206 may perform fordetermining whether adjustment criteria are met at block 776 aredescribed below in conjunction with FIG. 14 .

When adjustment criteria are met at block 776, control circuit 206determines what the adjustment should be (increase, decrease or noadjustment) at block 778 and performs the adjustment at block 780.Techniques for determining whether to increase, decrease or leave the A3window ending time unchanged are described below in conjunction withFIG. 14 . When control circuit 206 determines that adjustment criteriaare unmet (“no” branch of block 776), control circuit returns to block762 to determine the A4 sensing control parameter metrics over the nextset of N ventricular cycles as long as the operating mode still includesatrial event sensing. If the operating mode has changed, control circuit206 may suspend determining A4 sensing control parameter metrics used inadjusting the A3 window ending time and/or suspend adjusting the A3window ending time.

As described above in conjunction with FIG. 12 , the low A3 thresholdamplitude used to determine the A3 event time at block 764 may be basedon the low A4 sensing threshold amplitude. The low A4 sensing thresholdamplitude may be adjusted after every N ventricular cycles, e.g.,according to the techniques described above in conjunction with FIGS.9-11 . As such, before determining A3 event times during the next Nventricular cycles, control circuit 206 may determine whether the low A4sensing threshold amplitude has been adjusted after then current Nventricular cycles at block 775. If the low A4 sensing thresholdamplitude has been adjusted, e.g., based on the N ventricular cyclesthat have just ended (“yes” branch of block 775), control circuit 206may adjust the low A3 threshold amplitude at block 777. The low A3threshold amplitude is adjusted based on the adjusted low A4 sensingthreshold amplitude, e.g., to 75% of the new low A4 sensing thresholdamplitude or equal to the new low A4 sensing threshold amplitude.Otherwise, if the low A4 sensing threshold amplitude has not beenadjusted, control circuit 206 returns to block 762 without adjusting thelow A3 threshold amplitude.

FIG. 14 is a flow chart 776 of a method that may be performed by controlcircuit 206 at like-numbered block 776 of FIG. 13 for determining whenA3 window ending time adjustment criteria are met according to oneexample. At block 782, control circuit 206 may determine if the A3 timemetric is known. As described above, the A3 time metric may be unknownwhen two few A3 event times are available after N ventricular cycles.Too few A3 event times may be available when the A3 window is notstarted due to too many fast ventricular events or when the motionsignal does not cross the low A3 threshold amplitude during a thresholdnumber of the A3 windows.

If the A3 time metric is not known, control circuit 206 may determine ifthe cause of the unknown A3 time metric is due to too few A3 windowsduring the N ventricular events at block 784. Control circuit 206 maycompare the number of A3 windows that occurred over the N ventricularevents to a threshold number. When the number of A3 windows is less thana threshold number, e.g., less than 50% of the N ventricular cycles, theA4 sensing control parameter metrics may be insufficient to support anadjustment of the A3 window ending time since the A3 event time isunknown. Control circuit 206 may set the adjustment to zero at block 792in response to too few A3 windows being started during the N ventricularcycles. In some examples, control circuit 206 may count an A3 windowthat is started, even if it does not reach its ending time, e.g., due toa premature ventricular contraction sensed as an R-wave during the A3window. In other examples, control circuit may count an A3 window onlyif it is started and ends during the ventricular cycle.

When at least a threshold number of A3 windows occurred during the Nventricular cycles (“no” branch of block 784), control circuit 206 maydetermine at block 786 that the A3 time metric is not known due to toofew A3 event times being determined over the N ventricular cycles. Forexample, when an A3 event time has been determined in less than 50% ofthe N ventricular cycles), the A3 time metric may be unknown due to toofew A3 event times. In this case, the control circuit 206 may determinethat the A3 window ending time should be decreased at block 788. Sincethe motion signal seldom crosses the low A3 threshold amplitude,resulting if few A3 event times, the risk of oversensing A3 events islow. The A3 window ending time may be shortened to promote A4 sensingduring the A4 window.

Since the target A3 window ending time is not known when the A3 timemetric is not known, control circuit 206 may determine the adjustment tothe A3 window ending time at block 778 to be a predetermined decrement,e.g., 10 ms, 20 ms, 30 ms, 50 ms or other selected time interval. If thepredetermined decrement would cause the A3 window ending time to be lessthan a minimum A3 window ending time, the adjustment determined at block778 may be an adjustment to the minimum A3 window ending time (not lessthan the minimum A3 window ending time).

In other examples, rather than determining the A3 event time as unknown,and subsequently determine the A3 time metric to be unknown, when themotion signal does not cross the low A3 threshold amplitude, the A3event time may be set to a specified value, e.g., Y ms less than theminimum A3 window ending time, where Y ms may be 50 ms or otherpredetermined value. When the motion signal stays low, e.g., less thanthe low A3 threshold amplitude during the A3 window, the likelihood ofA3 event oversensing is small. As such, the A3 window ending time may beset to a relatively short time interval after the ventricular electricalevent with a small probability of oversensing A3 events as A4 events.Therefore, when the motion signal does not cross the low A3 thresholdamplitude in a given ventricular cycle, the A3 event time is set to arelatively short default time interval. The A3 time metric based on allA3 event times determined over the N ventricular cycles, which mayinclude cycles in which the A3 event time is set to the low defaultvalue, is then known. Control circuit 206 may use the A3 time metric forsetting a target A3 window ending time for determining the A3 windowending time adjustment. In this case, the criteria at block 786 fordetermining that the A3 time metric is not known due to too few A3 eventtimes may be omitted.

When control circuit 206 determines that the A3 time metric is known atblock 782 (“yes” branch), control circuit 206 may determine whether thepacing mode is an atrial tracking pacing mode and whether the A4 timemetric is known at block 790. If neither of these conditions are true,e.g., if the pacing mode is a non-atrial tracking pacing mode (e.g.,VDI, VVIR, etc.) or the A4 time metric is not known (e.g., due to twofew A4 windows or too few A4 event times determined over N ventricularcycles), control circuit 206 determines if the current operating mode isa VDI pacing mode at block 791.

As described below in conjunction with FIG. 18 , control circuit 206 mayoperate in a VDI pacing mode to adjust the A3 window ending time from aninitial, starting A3 window ending time value, e.g., at the time ofpacemaker implantation, before starting operations in an atrial trackingventricular pacing mode. If the VDI pacing mode is in effect at block791 control circuit 206 may advance to block 796. Otherwise, when the A4time metric is not known and the pacing mode is not an atrial trackingpacing mode or the VDI pacing mode (“no” branches of both blocks 790 and791), no adjustment is made to the A3 window ending time by controlcircuit 206 at block 792. Likewise, when the pacing mode is an atrialtracking ventricular pacing mode (e.g., VDD), but the A4 time metric isunknown (“no” branches of both blocks 790 and 791) no adjustment is madeto the A3 window ending time at block 792 since any adjustment may notcorrectly set the A3 window ending time earlier than the A4 event whenthe A4 event time is unknown.

When the operating mode is an atrial tracking pacing mode and the A4time metric is known (“yes” branch of block 790), control circuit 206may determine whether a sufficient difference between the A4 time metricand the A3 time metric exists to shift the A3 window ending time. Forexample, control circuit 206 may determine if the difference between theA4 time metric and the A3 time metric is less than a thresholddifference, e.g., less than 100 ms as an example. The thresholddifference may be a predetermined fixed value in some examples. In otherexamples, the threshold difference may be an adjustable or variablethreshold difference, e.g., scaled to the actual ventricular rate. Forinstance, the threshold difference may be a percentage of the actualventricular rate interval so that as the ventricular rate increases, thethreshold difference is decreased, and when the ventricular ratedecreases the threshold difference is increased.

If the difference between the A4 time metric and the A3 time metric isless than the threshold difference (“yes” branch of block 794), controlcircuit 206 determines that no adjustment is to be made to the A3 windowending time at block 792. In this case, enough A3 event times and enoughA4 event times are known from the N ventricular cycles so that both theA3 time metric and the A4 time metric are known. These known A3 and A4time metrics suggest that the current A3 window ending time is set to avalue that allows the A3 window to reliably end before the A4 event sothat the A3 and A4 events are separated by the ending time and eventtimes are reliably detected. However, the separation of the A3 and A4event signals is relatively small so no shifting of the A3 window endingtime is warranted.

When the difference between the A4 time metric and the A3 time metric isrelatively large, e.g., at least 100 ms or more, as determined bycontrol circuit 206 based on the comparison to the threshold differenceat block 794, control circuit 206 advances to block 796 (“no” branch ofblock 794). In this case, the time difference from A3 events to A4events is sufficiently large that a shift, either an increase ordecrease, in the A3 window ending time may be justified.

At block 796, control circuit 206 determines if the A3 window endingtime is less than or equal to the target A3 window ending time. Asdescribed above, in conjunction with FIG. 13 , control circuit 206 maydetermine the target A3 window ending time based on the A3 time metricduring a VDI pacing mode or based on the A3 time metric and the A4 timemetric during a VDD pacing mode. When the current A3 window ending timeis greater than the target A3 window ending time, control circuit 206determines that the A3 window ending time should be decreased at block788.

The current value of the A3 window ending time may be greater than thetarget A3 ending time when, for example, the A3 time metric isdetermined to be relatively short, e.g., when the A3 event times are setto default minimum values (e.g., the minimum A3 window ending time minus50 ms) due to the low A3 threshold amplitude not being crossed. In thiscase, the target A3 window ending time may be at or near the minimum A3window ending time. If control circuit 206 determines that the A3 windowending time should be decreased at block 788, the A3 window ending timeis adjusted toward or to the minimum A3 sensing window ending time. Inthis way, the A3 window is shortened when the likelihood of A3 eventoversensing is low due to relatively low amplitude A3 events (less thanthe low A3 threshold amplitude) or due to the A3 events occurring veryearly in the A3 window. Control circuit 206 shortens the A3 windowending time toward or to the minimum A3 window ending time to promotereliable A4 event sensing, even when the ventricular rate increases.Shortening the ending time of the A3 window effectively starts the A4window earlier, with the low A4 sensing threshold amplitude in effect,increasing the likelihood of sensing the A4 event.

When control circuit 206 determines that the A3 window ending timeshould be decreased at block 788, control circuit 206 may determine theadjustment at block 778 (of FIG. 13 ) based on the current value of theA3 window ending time. The A3 window ending time may be decreased by apredetermined decrement, e.g., 10 ms, 15 ms, 20 ms or other selecteddecrement but not less than the target A3 window ending time or theminimum available setting of the target A3 window ending time. Theminimum A3 window ending time may be 600 ms after the most recentpreceding ventricular electrical event as one example.

If the A3 window time is less than (or equal) to the target A3 windowending time as determined at bock 796 (“yes” branch), control circuit206 determines that the A3 window ending time should be increased atblock 798 toward (or held at) the target A3 window ending time (withoutexceeding the target A3 window ending time). It is to be understood thatwhen control circuit 206 determines that the A3 window ending time isequal to the target ending time, control circuit 206 determines that noadjustment to the ending time is to be made, e.g., the adjustment is 0ms.

When control circuit 206 determines that the A3 window ending timeadjustment is an increase at block 798 of FIG. 14 , control circuit 206may determine the adjustment at block 778 of FIG. 13 as a predeterminedincrement, e.g., 10 ms, 20 ms or other selected value, but not greaterthan the target A3 window ending time or a maximum allowable setting ofthe A3 window ending time, e.g., not greater than a maximum of 1000 msafter a most recent ventricular event. The minimum and maximum limits ofthe A3 window ending time may be programmable and tailored to a givenpatient and may be selected to be at least 100 ms apart in someexamples. For instance, the minimum to maximum range of the A3 windowending time may be 600 ms to 700 ms or 700 ms to 800 ms in a givenpatient. In another patient the minimum to maximum range may be larger,e.g., 700 ms to 1000 ms. At block 780 of FIG. 13 , control circuit 206adjusts the A3 window ending time according to the adjustment determinedat block 778, based on which adjustment criteria (increase, decrease orhold) were met at block 776 as described in conjunction with FIG. 14 .

FIG. 15 is a flow chart 800 of a method performed by control circuit 206for adjusting the high A4 sensing threshold amplitude according to oneexample. The high A4 sensing threshold amplitude is applied during theA3 window for detecting the A4 event when the A4 event signal is fusedwith the A3 event signal in the motion sensor signal. The high A4sensing threshold amplitude may also be referred to as the first orearly A4 sensing threshold since it is applied during the A3 window,before the low A4 sensing threshold amplitude, which may also bereferred to as the second or late A4 sensing threshold because it isapplied after the A3 window ending time. The high A4 sensing thresholdamplitude may be adjusted based on the A3 event amplitudes, e.g., A3event amplitude 760 shown in FIG. 12 , determined over N ventricularcycles.

When control circuit 206 is operating in a pacing mode that includesatrial event sensing, e.g., a VDD or VDI pacing mode as determined atblock 802, control circuit 206 determines the A3 event amplitude foreach ventricular cycle at block 804 until N ventricular cycles haveelapsed (as determined at block 806). It is to be understood that the A3event amplitude, as well as other A4 sensing control parameter metrics,may be determined during or at the end of each ventricular cycle. Inother examples, control circuit 206 may buffer the motion sensor signalin memory 210 for N ventricular cycles. Upon expiration of the Nventricular cycles, control circuit 206 may determine the A3 eventamplitude (and/or other A4 sensing control parameter metrics) from themotion sensor signal buffered in memory 210. As described above, controlcircuit 206 may not determine an A3 event amplitude when an A3 window740 (see FIG. 12 ) does not occur, e.g., due to an R-wave being sensedduring the blanking period 730. When the A3 window 740 is started,control circuit 206 determines the maximum motion sensor signalamplitude during the A3 window as the A3 event amplitude.

In some examples, control circuit 206 may determine the A3 eventamplitude only during ventricular cycles that include an A4 eventdetected during the A4 window. Alternatively, the A3 event amplitude maybe determined for all ventricular cycles but ignored for the purposes ofdetermining an A3 amplitude metric at block 808 when the A4 event is notdetected during the A4 window. In some examples, A3 event amplitudesdetermined only during ventricular cycles that include a normal A4 eventdetection are used by control circuit 206 in adjusting the high A4sensing threshold amplitude as described below. A normal A4 eventdetection is an A4 event detected at least a threshold time interval,e.g., 50 to 100 ms, after the A3 window ending time. The A3 eventamplitude may more accurately reflect the amplitude of a true A3 eventwhen the A4 event is known to be detected later during the A4 window.When the A4 event is detected early, fusion of the A3 and A4 events maybe occurring, so that the A4 event is contributing to the A3 eventamplitude determined by control circuit 206. As such, control circuit206 may reject the A3 event amplitude that occurs during a ventricularcycle when the A4 event is detected during the A3 window, as an early A4event within a threshold time interval of the A3 window ending time, ornot detected at all.

In still other examples, the A3 event amplitude may be determined forall ventricular cycles, however different A3 event amplitude metrics maybe determined by control circuit 206 according to the timing of the A4event detection. As such, control circuit 206 may store the A3 eventamplitudes in different buffers in memory 210 based on when the A4 eventis detected. For example, control circuit 206 may determine and store A3event amplitudes for all ventricular cycles in memory 210 and may flagor store in a different buffer the A3 event amplitudes determined forventricular cycles that include only normal A4 event detections.

After N ventricular cycles have elapsed (block 806), control circuit 206determines an A3 amplitude metric at block 808 based on the individuallydetermined A3 event amplitudes. The A3 amplitude metric may be a mean,median, maximum or nth highest A3 event amplitude, as examples. When theA3 event amplitudes are determined over eight ventricular cycles, the A3amplitude metric may be determined at the fourth highest A3 eventamplitude as one example. If fewer than four A3 event amplitudes areknown, e.g., due to fewer than four A3 windows being started during theeight ventricular cycles or the motion signal not crossing the low A3threshold amplitude, the A3 amplitude metric may be determined to beunknown.

As described above, the A3 event amplitude may be determined as themaximum motion signal amplitude during the A3 window that is greaterthan the low A3 threshold amplitude, which may be set to a percentage ofthe low A4 sensing threshold amplitude. If the motion signal does notcross the low A3 threshold amplitude during the A3 window, the A3 eventamplitude may be unknown for that ventricular cycle. As the low A4sensing threshold amplitude is adjusted (e.g., as often as every Nventricular cycles), the low A3 threshold amplitude may be adjustedbased on the adjusted low A4 sensing threshold amplitude. As such, thelow A3 threshold amplitude is adjusted with the low A4 sensing thresholdamplitude but the high A4 sensing threshold amplitude applied during theA3 window may be adjusted less frequently as described below.

As indicated above, the A3 amplitude metric may be determined based onlyon A3 event amplitudes determined for ventricular cycles having normalA4 event detections. For example, the A3 amplitude metric may be set tothe maximum A3 event amplitude or a predetermined percentile or nthhighest of the A3 event amplitudes associated with normal A4 eventdetection cycles. In other examples, the A3 amplitude metric may bedetermined based on a combination of A3 event amplitudes determined forventricular cycles having normal A4 event detections and A3 eventamplitudes determined for all ventricular cycles. For example, the A3amplitude metric may be set to a percentile of all A3 event amplitudesthat are greater than the A3 event amplitudes associated with normal A4event detection cycles.

At block 810, control circuit 206 determines a target high A4 sensingthreshold amplitude based on the A3 amplitude metric determined at block808. The target high A4 sensing threshold amplitude may be determinedbased on a combination of the current low A4 sensing threshold amplitudeand the A3 amplitude metric in some examples. For instance, controlcircuit 206 may determine the target high A3 sensing threshold amplitudeas the sum of the current low A4 sensing threshold amplitude plus the A3amplitude metric multiplied by a threshold factor. The threshold factormay be 1, 1.2, 1.5, 1.8, or other selected value. In some examples, thetarget high A4 sensing threshold amplitude is set to the low A4 sensingthreshold amplitude plus 1.5 times the A3 amplitude metric plus anoffset. Since a true A4 event signal during the A3 window may be fusedwith the A3 event signal, the fused A3/A4 amplitude may be a summationof the two individual events. Therefore, the target high A4 sensingthreshold amplitude may be set based on a combination of the low A4sensing threshold amplitude (or minimum detected A4 amplitude) and theA3 amplitude metric. The offset may be set to zero but may be set toother positive or negative values to fine tune the high A4 sensingthreshold amplitude. In other examples, the target high A4 sensingthreshold amplitude may be set to the maximum programmable low A4sensing threshold amplitude plus the A3 amplitude metric multiplied by aselected factor, e.g., 1.5. When the A3 amplitude metric is unknown, thetarget high A4 sensing threshold amplitude may be determined to beunknown at block 810.

At block 812, control circuit 206 may adjust a high A4 thresholdadjustment counter based on the target value determined at block 810.When the target high A4 sensing threshold amplitude is greater than thecurrent high A4 sensing threshold amplitude, the adjustment counter isincreased by 1. When the target high A4 sensing threshold amplitude isless than the current high A4 sensing threshold amplitude, controlcircuit 206 decreases the high A4 adjustment counter by 1. If the targetvalue and the current value of the high A4 sensing threshold amplitudeare equal, control circuit 206 does not change the value of the high A4adjustment counter at block 812.

At block 814, control circuit 206 determines if the adjustment counterhas reached a threshold. The adjustment counter may be increased up to apredetermined maximum value and decreased down to a predeterminedminimum value. In other examples, instead of utilizing an adjustmentcounter, control circuit 206 may set a timer or counter for adjustingthe high A4 sensing threshold amplitude on a scheduled basis, e.g., onceper hour, once per four hours, once per eight hours, once per twelvehours, once per day or other selected frequency. When the adjustmentcounter reaches the maximum or minimum threshold value, or when ascheduled adjustment time is reached, control circuit 206 may determineif any other required high A4 sensing threshold adjustment criteria aremet at block 815.

In some examples, control circuit 206 may determine if heart rate and/orthe A3 window detection count is/are less than an adjustment thresholdat block 815. During high heart rates, the large amplitude of the fusedA3/A4 events will be determined as the A3 event amplitude and contributeto the A3 amplitude metric. Since the target value of the high A4sensing threshold amplitude includes the A3 amplitude metric (incombination with the low A4 sensing threshold amplitude), the targetvalue will be increased during periods of fused A3/A4 events. Thiselevated target value could, under some circumstances, cause the high A4sensing threshold amplitude to be incremented above the fused A3/A4amplitude and lead to undersensing of the fused A3/A4 events. As such,in some examples, the high A4 sensing threshold may be held at a currentvalue (no adjustment) when sustained sensing in the A3 window isoccurring. Accordingly, adjustment criteria applied at block 815 mayrequire that the A3 window detection count is less than a threshold forone or more sets of N ventricular cycles and/or the actual ventricularrate is less than a threshold rate, e.g., less than 90 beats per minuteindicating in order for adjustment criteria to be met. When the A3window detection count is greater than the threshold count, e.g.,greater than 3, and/or the heart rate is greater than 90 beats perminute, there is a likelihood of fused A3/A4 events contributing to anincreased A3 event amplitude and an increased target high A4 sensingthreshold amplitude. Control circuit 206 withholds adjusting to the highA4 sensing threshold amplitude under these conditions. The frequency ofthe adjustments to the high A4 sensing threshold amplitude may becontrolled, therefore, by setting the maximum and minimum adjustmentcounter thresholds and may be further reduced when the A3 windowdetection count reaches a threshold value over one or more sets of Nventricular cycles.

When the adjustment counter reaches the maximum value and any otheradjustment criteria are met at block 815, the high A4 sensing thresholdamplitude may be increased at block 816. When the adjustment counterreaches the minimum value, the high A4 sensing threshold amplitude maybe decreased at block 816. The range of the maximum and minimum valuesof the adjustment counter controls the frequency of adjustments made tothe high A4 sensing threshold. The high A4 sensing threshold amplitudeis increased or decreased only after the high A4 adjustment counter hasprogressively increased or decreased a threshold number of times. Forexample, the high A4 adjustment counter may be increased by one eachtime the target high A4 sensing threshold amplitude is greater than thecurrent high A4 sensing threshold amplitude up to a maximum count ofpositive eight. When the maximum positive count of eight is reached(block 814) due to the target high A4 sensing threshold amplitude beingconsistently greater than the current setting of the high A4 sensingthreshold, the high A4 sensing threshold may be increased by controlcircuit 206 at block 816.

The adjustment counter may be decreased by one each time the target highA4 sensing threshold amplitude is less than the current high A4 sensingthreshold amplitude down to a minimum of negative eight, in someexamples. When the threshold count of negative eight is reached (block814), control circuit 206 may decrease the high A4 sensing thresholdamplitude at block 816. In this way, the high A4 sensing thresholdamplitude may be adjusted less often than the low A4 sensing thresholdamplitude but will slowly track changes in the low A4 sensing thresholdamplitude and the A3 amplitude metric. Other thresholds than the valuesof positive eight and negative eight may be applied to the high A4adjustment counter, such as ±2, +4, +6 or other selected value, tocontrol how often the high A4 sensing threshold amplitude is adjustedrelative to the low A4 sensing threshold amplitude. Other techniques,other than the adjustment counter described herein, such as a timer orventricular cycle counter may be used to limit or control the frequencyat which the high A4 sensing threshold amplitude is adjusted.

When the adjustment counter reaches the maximum threshold value at block814, control circuit 206 increases the high A4 sensing thresholdamplitude toward the target high A4 sensing threshold amplitude at block816. The adjustment may be a predetermined increment, e.g., 1 ADC unitor 0.12 m/s², 0.2 m/s², 0.3 m/s², or other selected increment. In otherexamples, the high A4 sensing threshold amplitude is adjusted to thetarget value or adjusted by a scaled increment based on the differencebetween the target value and the current high A4 sensing thresholdamplitude.

When the adjustment counter reaches the minimum threshold value at block814, control circuit 206 decreases the high A4 sensing thresholdamplitude toward the target value at block 816. The high A4 sensingthreshold amplitude may be decreased to the target value, by apredetermined decrement, e.g., 0.3 m/s², or by a scaled or adjustabledecrement. The high A4 sensing threshold amplitude may be adjustedwithin a specified range, e.g., up to an A4 sensing threshold upperlimit and down to a high A4 sensing threshold lower limit, which may beadjustable and based on or equal to the low A4 sensing threshold. Afteradjusting the high A4 sensing threshold amplitude, the adjustmentcounter is reset to zero (or a middle value between its maximumthreshold and minimum threshold) at block 820.

If the adjustment counter has not reached a maximum or minimum thresholdvalue at block 814 (“no” branch), control circuit 206 may return toblock 802 without adjusting the high A4 sensing threshold amplitude.Control circuit 206 may proceed with determining the A3 amplitude metricfor the next N ventricular cycles, as long as the pacing mode includesatrial sensing. In some examples, before returning to block 802, controlcircuit 206 may check at block 818 whether the high A4 sensing thresholdor the low A4 sensing threshold amplitude has been reprogrammed by auser to a new value. If a programming change has been made to either thehigh A4 sensing threshold or the low A4 sensing threshold, theadjustment counter may be reset at block 820 to a starting value of zeroor to a middle value of its range between a minimum threshold and amaximum threshold.

At block 819, control circuit 206 may determine if the low A4 sensingthreshold amplitude has been adjusted after the most recent Nventricular cycles. If so, the low A3 threshold amplitude is adjusted atblock 821. As described above, the low A3 threshold amplitude may be setbased on the low A4 sensing threshold amplitude, e.g., to 75% or anotherspecified percentage of the low A4 sensing threshold amplitude. Alsodescribed above in conjunction with FIG. 12 , the A3 event amplitudedetermined at block 804 may be determined by control circuit 206 to bethe maximum amplitude of the motion signal during the A3 window that isgreater than the low A3 threshold amplitude. Accordingly, in someexamples the low A3 threshold amplitude may be adjusted in response toany programming change of the low A4 sensing threshold amplitude and inresponse to any adjustment of the low A4 sensing threshold amplitudeafter N ventricular cycles in order to enable appropriate determinationof the A3 event amplitude.

Additionally or alternatively, control circuit 206 may compare the highA4 sensing threshold amplitude to the low A4 sensing threshold amplitudeat block 822 to verify that, if the low A4 sensing threshold amplitudeis incremented, the high A4 sensing threshold amplitude is always atleast equal to or greater than the low A4 sensing threshold amplitude.If control circuit 206 determines that the low A4 sensing thresholdamplitude has been adjusted to be higher than the high A4 sensingthreshold amplitude at block 822, control circuit 206 may adjust thehigh A4 sensing threshold amplitude to be at least equal to or greaterthan the low A4 sensing threshold amplitude at block 824. Thisadjustment at block 822 may be before the adjustment counter reaches amaximum or minimum threshold value. Control circuit 206 may return toblock 802 to continue determining the A3 amplitude metric for updatingthe target high A4 sensing threshold amplitude after verifying that theoperation mode still includes atrial event sensing.

Generally, the high A4 sensing threshold amplitude will be greater thanthe A3 event amplitude since it is set based on a combination of the A3amplitude metric and a multiple of the low A4 sensing thresholdamplitude. The high A4 sensing threshold amplitude set in this wayavoids A3 event oversensing during the A3 window. When the heart rateincreases, however, and the A3 event signal and the A4 event signalbecome fused, the fused A3/A4 event signal may cross the high A4 sensingthreshold amplitude for appropriately sensing the A4 event. By adjustingthe high A4 sensing threshold amplitude less frequently than the low A4sensing threshold amplitude, oversensing of the A3 event is avoidedwhile allowing sustained periods of sensing the fused A3/A4 event signalin the A3 window. The adjustment counter maximum and minimum thresholdsmay be set to limit adjustments to the high A4 sensing thresholdamplitude of about 0.1 m/sec² per minute, as an example.

In the illustrative examples disclosed herein, control circuit 206determines A4 sensing control parameter metrics from the motion sensorsignal over non-overlapping sets of N consecutive ventricular cycles.Control circuit 206 determines an adjustment (increment, decrement, orno adjustment) to an A4 sensing control parameter based on the controlparameter metrics after each set of N ventricular cycles (or after theadjustment counter reaches a threshold in the case of the high A4sensing threshold amplitude). It is to be understood, however, thatcontrol circuit 206 may update A4 sensing control parameter metricsaccording to other schedules or frequencies, e.g., on a beat-by-beatbasis or after a rolling number of overlapping N ventricular cycles. Forinstance, A4 sensing control parameter metrics may be updated afterevery four ventricular cycles based on the most recent eight ventricularcycles. It is recognized that the A4 sensing control parameter metricsmay be updated according to a variety of schedules based on a variety ofselected numbers of ventricular cycles (e.g., every one, two, three,four, five, eight, twelve, sixteen, twenty-four, or any other selectednumber of cycles).

Furthermore, it is recognized that the schedule and/or number ofventricular cycles required for updating one A4 sensing controlparameter metric may be different than the schedule and/or number ofventricular cycles required for updating another A4 sensing controlparameter metric. Different control parameter metrics may be updated atdifferent frequencies. As described in conjunction with FIG. 15 , thehigh A4 sensing threshold amplitude is adjusted less frequently than thelow A4 sensing threshold amplitude and the A3 window ending time byimplementing an adjustment counter that tracks differences in the targethigh A4 sensing threshold amplitude and the current high A4 sensingthreshold amplitude. It is understood that various timers and/orcounters may be implemented to control the schedule and frequency ofdifferent A4 sensing control parameter adjustments.

The scheduled frequency or number of ventricular cycles at which a givenA4 sensing control parameter is adjusted may be fixed or adjustable. Forexample, if a given A4 sensing control parameter is not adjusted for athreshold time interval, e.g., within a specified number of sets of Nventricular cycles, due to adjustment criteria not being met, the numberN may be increased so that determinations of A4 sensing controlparameter metrics and A4 sensing control parameter adjustments areperformed less often. To illustrate, if control circuit 206 determinesthat the low A4 sensing threshold amplitude has not been adjusted inresponse to the last six, eight, ten or other specified number of Nventricular cycles, the number N may be doubled, e.g., from eight tosixteen ventricular cycles. The A4 sensing control parameter metricsdetermined for use in adjusting the low A4 sensing threshold amplitudemay be determined after every 16 ventricular cycles instead of everyeight ventricular cycles, as an example.

When control circuit 206 determines that the low A4 sensing thresholdamplitude is adjusted twice in a row (or other specified number oftimes) after 16 ventricular cycles, the number N may be reduced, e.g.,back to 8 ventricular cycles, to allow more frequent adjustments to thelow A4 sensing threshold amplitude. In some examples, the number ofventricular cycles used to determine A4 sensing control parametermetrics and adjust an A4 sensing control parameter may be increased oneor more times based on the stability of the A4 sensing control parameterand may be reduced one or more times based on the frequency ofadjustments to the A4 sensing control parameter. For example, at block304 in FIG. 6 control circuit 206 may adjust N, the number ventricularcycles, based on how often one or more A4 sensing control parametermetrics are adjusted. Adjustments to the specified number of ventricularcycles may allow current drain of power source 214 required by controlcircuit 206 to perform processing and analysis of the motion signal tobe conserved during periods of stable motion signal and cardiac rhythm(when frequent adjustments are not needed) while providing adjustmentsat increased frequency when the motion signal and or heart rhythm ischanging due to changing patient conditions. As such, it is to beunderstood that control circuit 206 may adjust N, the number ofventricular cycles, used to determine the A4 sensing control parametermetrics when determining if N ventricular cycles are reached, e.g., atblock 506 of FIG. 9 , block 768 of FIG. 13 , and block 806 of FIG. 15 .

Among the A4 sensing control parameter metrics that may be determined bycontrol circuit 206 for adjusting A4 sensing control parameters, with nolimitation intended, are the applicable cycle count (ventricular cyclesthat include an A4 window), detected A4 count, minimum detected A4amplitude or other A4 amplitude metric, early A4 count, normal A4 count,A3 window detection count (number of A4 events sensed during the A3window), low A3 threshold amplitude, A3 time metric, A4 time metric,target A3 window ending time, A3 amplitude metric, and target high A4sensing threshold amplitude, all of which are described above inconjunction with the accompanying figures.

The A4 sensing control parameter adjustments described herein involvedetermining A4 sensing control parameter metrics during ventricularcycles that may begin with a sensed R-wave or a ventricular pacingpulse. In some examples, control circuit 206 may be configured todetermine a first set of A4 sensing control parameter metricscorresponding to ventricular cycles starting with a sensed R-wave anddetermine a second set of A4 sensing control parameter metricscorresponding to ventricular cycles starting with a ventricular pacingpulse. In this way, one set of A4 sensing control parameters may beadjusted for sensing the A4 event following a sensed R-wave and a secondset of A4 sensing control parameters may be adjusted for sensing the A4event following a ventricular pacing pulse. For instance, a post-senseA3 window ending time may be adjusted based on the A3 time metric andthe A4 time metric determined following sensed R-waves. This post-senseA3 window ending time may be applied following sensed R-waves. Apost-pace A3 window ending time may be adjusted based on the A3 timemetric and the A4 time metric determined following ventricular pacingpulses and applied following ventricular pacing pulses. Likewise,post-pace and post-sense high A4 sensing threshold amplitudes andpost-pace and post-sense low A4 sensing threshold amplitudes may beadjusted separately based on respective post-pace A4 sensing controlparameter metrics and post-sense A4 sensing control parameter metrics.

Examples presented herein indicate that the A4 sensing control parameteradjustment methods may be performed during specified pacing operatingmodes for use in controlling A4 sensing during atrial trackingventricular pacing modes. In some instances, the telemetry circuit 208of pacemaker 14 may be receiving or sending telemetry signals orperforming other temporary operations. Determining A4 sensing controlparameter metrics and adjusting of A4 sensing control parameters may besuspended by control circuit 206 during at least some temporaryoperations, e.g., when adjustments to the A4 sensing control parametersmay interfere with or confound the temporary operation or otherfunctions of pacemaker 14 during the temporary operation.

It further is contemplated that the A4 sensing control parameters may beadjusted by a medical device during and for use in a sensing operatingmode that does not necessarily include ventricular pacing. A4 sensingmay be performed for monitoring an atrial rate or rhythm, discriminatingventricular tachyarrhythmias from supraventricular tachyarrhythmias orother applications that require atrial systolic event sensing but do notnecessarily include ventricular pacing. As such the techniques disclosedherein are not limited to implementation for use in conjunction withventricular pacing or in a ventricular pacemaker or other medical devicethat necessarily includes ventricular pacing operating modes.

FIG. 16 is a flow chart 900 of a method that may be performed by controlcircuit 206 for adjusting the high A4 sensing threshold amplitudeaccording to another example. When control circuit 206 is operating inan operating mode that includes atrial event sensing, as determined atblock 902, A4 sensing control parameter metrics may be determined fromthe motion signal for use in adjusting the high A4 sensing thresholdamplitude. At block 904, control circuit 206 determines A3 eventamplitudes. In the method of FIG. 16 , control circuit 206 may adjustthe high A4 sensing threshold amplitude at scheduled adjustment timeintervals, rather than based on an adjustment counter as described inconjunction with FIG. 15 . As such, control circuit 206 may determine A3event amplitudes for each ventricular cycle until the adjustment timeinterval expires (at block 908).

During the adjustment time interval, which may be several minutes, hoursor days in various examples, the A3 event amplitude may be determined atblock 904 for each ventricular cycle having an A3 window by determiningthe maximum amplitude of the motion sensor signal during the A3 window.In some examples, the A3 event amplitude is determined for a givenventricular cycle only when the A3 window reaches its ending time. Inother examples, the A3 event amplitude is determined for all ventricularcycles in which the A3 window is started. As described above inconjunction with FIG. 12 , the maximum amplitude of the motion signalmay be required to be greater than the low A3 threshold in order to bedetermined as an A3 event amplitude.

At block 906, control circuit 206 may store the A3 event amplitudes inhistogram bins allocated in memory 210. In some examples, the A3 eventamplitudes are stored in two different histograms, each having multiplehistogram bins assigned to different amplitude values or ranges. Alldetermined A3 event amplitudes may be stored in one histogram,regardless of timing of A4 event detections. In the second histogram, A3event amplitudes determined only for ventricular cycles including anormal A4 event detection may be stored. When an A4 event is sensedduring the A4 window, or at least a threshold time interval later thanthe A3 window ending time, the ventricular cycle may be determined to bea “normal” A4 event cycle since the A4 event was not sensed early orduring the A3 window. During these normal A4 event cycles, the maximumamplitude during the A3 window is most likely the A3 event, not a fusedA3/A4 event, and therefore representative of the true A3 event amplitudewith a high degree of confidence. As such, one histogram may be a normalA4 event histogram allocated for storing A3 event amplitudes for allventricular cycles having a normal A4 event sensed at least a thresholdtime interval (e.g., 0-100 ms) later than the A3 window ending time.

In another example, in one histogram control circuit 206 may store onlyA3 event amplitudes that are determined to occur within a predeterminedrange of the A3 time metric or a currently determined median (or similarmetric) of A3 event times determined over the adjustment time interval.For example, when the maximum amplitude during the A3 window is within10 to 20 ms of the median A3 event time, control circuit 206 may storethis maximum amplitude in the corresponding bin of the histogram. Inthis way, a portion of the A3 event amplitudes each determined as amaximum amplitude of the motion signal during the A3 window and within athreshold time range of an A3 time metric may be identified for use indetermining an A3 amplitude metric. This A3 amplitude metric based onmaximum amplitudes within a threshold time range of the A3 time metricmay be more likely to correspond to true A3 events.

At block 908, control circuit 206 may determine when a scheduledadjustment time interval for adjusting the high A4 sensing thresholdamplitude has expired. In some examples, the high A4 sensing thresholdamplitude is adjusted once every 24 hours. In other examples, thescheduled adjustment time may be more often or less often and may be setto a variable time interval by control circuit 206 based on the timesince pacemaker 14 implant, relative changes in A3 event amplitudes, A4event amplitudes, a change in a target high A4 threshold value, thedifference between a target high A4 threshold value and the currentsetting of the high A4 threshold, or other factors. If the adjustmenttime interval that is scheduled for adjusting the high A4 sensingthreshold has not expired, control circuit 206 returns to block 902 tocontinue accumulating A3 event amplitudes for storing in histogram(s) inmemory 210.

If the operating mode has changed from an atrial event sensing mode,control circuit 206 may wait until the atrial event sensing moderesumes. The A3 event amplitudes that have accumulated in memory 210 maybe retained until the scheduled time for adjusting the high A4 sensingthreshold arrives, with additional A3 event amplitudes being added tothe histogram(s) in memory 210 whenever the operating mode includesatrial event sensing. Alternatively, the A3 event amplitudes may becleared from memory 210 if the operating mode changes to a mode withoutatrial event sensing. Collection of A3 event amplitudes in memory 210may be restarted the next time control circuit 206 begins operating inan operating mode that includes atrial event sensing.

When control circuit 206 determines that it is time to adjust the highA4 sensing threshold amplitude at block 908, control circuit 206 maydetermine if the histogram(s) are sufficiently populated at block 909.In some instances, there may be insufficient ventricular cycle lengthshaving a normal A4 event detection resulting in a low or sparselypopulated histogram of the A3 event amplitudes for normal A4 eventcycles. A threshold number of A3 event amplitudes may be required to bestored in the normal A4 event histogram in order to utilize theaccumulated data for determining an A3 amplitude metric as describedbelow. When control circuit 206 determines that less than a thresholdnumber of the ventricular cycles are identified as normal A4 eventventricular cycles at block 909, control circuit 206 may withholddetermining the A3 amplitude metric. When insufficient data exists,e.g., less than a threshold number of stored A3 event amplitudes fornormal A4 event ventricular cycles, control circuit 206 may return toblock 902 to continue accumulating A3 event amplitudes without adjustingthe high A4 sensing threshold amplitude. In various examples, the highA4 sensing threshold amplitude may be adjusted when the next adjustmenttime interval expires or when a threshold number of A3 event amplitudespopulate the normal A4 event histogram, or whichever occurs first.

In some examples, the limited number of A3 event amplitudes stored inthe normal A4 event histogram may be due to A3 event oversensing. Asdescribed above, e.g., in conjunction with FIG. 10 , when the A3 eventis oversensed as the A4 event, e.g., due to the high A4 sensingthreshold amplitude being set too low, the A4 window is not started.This precludes determination and storage of the A3 event amplitude inthe normal A4 event histogram. As such, control circuit 206 maydetermine if A3 event oversensing criteria are met at block 916 inresponse to less than a threshold number of A3 event amplitudes beingstored in the normal A4 event histogram (as determined at block 909).

The A3 event oversensing criteria may be met at block 916 when greaterthan a threshold number of A3 event amplitudes are stored in a histogramfor all ventricular cycles but less than a threshold number ofamplitudes are stored in the histogram for only the normal A4 eventventricular cycles. In other examples, control circuit 206 may track thenumber of applicable cycles (the number of cycles having an A4 windowstarting after the expiration of the A3 window). When fewer than athreshold number of ventricular cycles over the scheduled adjustmenttime interval are applicable cycles, A3 event oversensing criteria maybe met at block 916. In still other examples, control circuit 206 maydetermine the early A4 count (the number of A4 events sensed within athreshold time interval of the ventricular window ending time) and/orthe A3 window detection count over the adjustment time interval. Wheneither or the sum of these A3 window and early A4 counts is greater thana threshold, the A3 event oversensing criteria may be met at block 916.For instance, control circuit 206 may compare the early A4 count, the A3window detection count, or the sum of both of these counts to theapplicable cycle count or to the detected A4 count determined over theadjustment time interval. When the ratio or difference between the A3window detection count, early A4 count or sum of both and the applicablecycle count (or the detected A4 count) is greater than a threshold,control circuit 206 may determine that the A3 event oversensing criteriaare met at block 916.

If A3 event oversensing criteria are not met (“no” branch of block 916),control circuit 206 may return to block 902. When the A3 eventoversensing criteria are met, control circuit 206 may increase the highA4 sensing threshold at block 918, e.g., by a predetermined increment.By increasing the high A4 sensing threshold amplitude, oversensing of A3events may be reduced or avoided so that the histogram storing A3 eventamplitudes determined from normal A4 event ventricular cycles may besufficiently populated over the next scheduled adjustment time interval.In some examples, control circuit 206 increases the high A4 thresholdamplitude at block 918 in response to less than a threshold number of A3event amplitudes stored in the normal A4 event histogram (as determinedat block 909) without requiring any additional A3 event oversensingcriteria to be met at block 916.

When the time to adjust the high A4 sensing threshold is reached atblock 908 and the histogram(s) are sufficiently populated (block 909),control circuit 206 determines an A3 amplitude metric at block 910 basedon the A3 event amplitudes stored in memory 210. In some examples, theA3 amplitude metric may be a predetermined percentile of A3 eventamplitudes stored for the ventricular cycles that include a normal A4event detection. For instance, the A3 amplitude metric may be determinedto be the 80th, 85th, 90th, or 95th percentile of the A3 eventamplitudes determined from ventricular cycles including a normal A4event detection.

At block 912, control circuit 206 may determine a target high A4 sensingthreshold amplitude. The target high A4 sensing threshold amplitude maybe determined based on the A3 amplitude metric. In other examples, thetarget high A4 sensing threshold amplitude is determined based on the A3amplitude metric and all accumulated A3 event amplitudes stored inmemory 210 over the scheduled adjustment time period, regardless ofwhether the A4 event is detected early or late in the associatedventricular cycles. In one example, the target high A4 sensing thresholdamplitude is determined as a predetermined percentile of all A3 eventamplitudes that are greater than the A3 amplitude metric. Examplemethods for determining the A3 amplitude metric and a target high A4sensing threshold amplitude based on histograms of A3 event amplitudesare described below in conjunction with FIG. 17 .

At block 914, control circuit 206 adjusts the high A4 sensing thresholdamplitude toward the target high A4 threshold. In some examples, controlcircuit 206 adjusts the high A4 sensing threshold amplitude directly tothe target value determined at block 912. In other examples, controlcircuit 206 may adjust the high A4 sensing threshold amplitude towardthe target value by a predetermined adjustment value, which may be anincrement or decrement as needed to adjust toward the target value. Ifthe current high A4 sensing threshold amplitude is equal to the targetvalue, no adjustment is made.

The adjustment value applied as an increment or decrement to the currenthigh A4 sensing threshold amplitude to adjust toward the target high A4sensing threshold may be fixed or variable. For example, the adjustmentvalue may be fixed at 0.2 m/s², 0.4 m/s², 0.6 m/s², 0.8 m/s² or otherselected value. In other examples, the adjustment value may be variable.For example, the adjustment increment/decrement may be set by controlcircuit 206 based on the time since pacemaker 14 implant. During thefirst days, weeks or months after implant, the adjustmentincrement/decrement may be set larger to enable adjustment toward thetarget high A4 threshold to occur more rapidly. The size of theadjustment increment/decrement may be decreased gradually or in one stepafter a specified time since pacemaker implant, e.g. after one week, onemonth or other selected time period. In other examples, the adjustmentvalue of the increment/decrement may be variably adjusted by controlcircuit 206 based on the difference between the target high A4 thresholdand the current setting of the high A4 threshold. In still otherexamples, the adjustment value may be adjusted based on the scheduledadjustment time interval that determines the frequency of high A4sensing threshold adjustments. For instance, a smallerincrement/decrement may be used when the scheduled adjustment timeintervals are relatively short, e.g., less than every 12 hours, and alarger increment/decrement may be used when the schedule adjustment timeintervals are relatively longer, e.g., every 12 hours or more.

After making the appropriate adjustment at block 914, control circuit206 may clear the histogram bins (or a portion of the histogram bins)and return to block 902 to restart accumulating A3 event amplitudes whenthe operation mode includes atrial event sensing. Control circuit 206may schedule the next time to adjust the high A4 sensing thresholdamplitude, e.g., at a fixed time interval or adjustable time interval.As indicated above, control circuit 206 may schedule the time foradjusting the high A4 threshold amplitude according to a variableadjustment time interval, which control circuit 206 may set based on adifference between the current target high A4 sensing thresholdamplitude a previously determined target high A4 sensing thresholdamplitude, the time since implant of pacemaker 14, the differencebetween the target high A4 sensing threshold amplitude and the currentsetting of the high A4 sensing threshold amplitude, or other factors.

The histogram(s) storing the A3 event amplitudes may be clearedcompletely after adjusting the high A4 sensing threshold amplitude or ona first in first out basis such that some portion of the most recent A3event amplitudes may remain in the histogram bin(s) and be included inthe next determinations of the A3 amplitude metric and target high A4sensing threshold amplitude. For example, the A3 event amplitudehistogram bins may store data over a first time interval, e.g., 24 hoursof data, but the high A4 sensing threshold may be adjusted after asecond time interval which may be shorter than the first time interval,e.g., every 12 hours based on the most recent 24 hours of data. Theoldest 12 hours of A3 event amplitude data may be cleared afteradjusting the high A4 sensing threshold amplitude at block 914, and themost recent 12 hours of A3 event amplitude data may be saved in thehistograms and compiled with the next 12 hours of new data.Alternatively, multiple histograms may be allocated in memory 210 toenable storing data in histograms over different overlapping timeperiods. A3 event amplitude data stored over a relatively longer periodof time may represent a greater variation in heart rates, physicalactivity or other factors that may influence the timing of A4 eventsduring ventricular cycles and/or the motion signal amplitude. Adjustingthe high A4 sensing threshold amplitude at relatively shorter adjustmenttime intervals based long a longer data collection time interval allowsthe threshold amplitude to appropriately track any changes in the motionsignal amplitude more frequently.

FIG. 17 is a diagram 950 of histograms 952 and 962 of A3 eventamplitudes that may be determined by control circuit 206 and stored inmemory 210 for use in adjusting the high A4 sensing threshold amplitudeaccording to some examples. The A3 event amplitudes are plotted alongthe horizontal axis as acceleration measured in m/s² in bin ranges of0.1 m/s². The frequency of occurrence of each amplitude bin range isplotted along the y-axis.

The top histogram 952 is a histogram of A3 event amplitudes determinedfrom only ventricular cycles including normal A4 event detections. Inthese ventricular cycles, the A4 event is detected in response to themotion signal crossing the low A4 sensing threshold amplitude later thana threshold time interval, e.g., 0 to 100 ms, after the A3 window endingtime (without crossing the high A4 sensing threshold amplitude duringthe A3 window). In some examples, all ventricular cycles having an A4event sensed at least 50 ms later than the A3 window ending time areclassified as normal A4 event cycles by control circuit 206. In theseventricular cycles, the determined A3 event amplitude is unlikely to beinfluenced by an A4 event (that occurs later in the ventricular cyclethan the A3 event) and more likely corresponds to a true A3 event thatis not fused with an A4 event.

Control circuit 206 may determine the A3 amplitude metric at block 910of FIG. 16 as a predetermined percentile of the A3 event amplitudesincluded in normal A4 event histogram 952. In the example shown, the A3amplitude metric is determined as the 95th percentile of the A3 eventamplitudes stored in normal A4 event histogram 952. The line 954 marksthe 95th percentile of the A3 event amplitudes included in histogram 952and is referred to hereafter as the A3 amplitude metric 954 for thisillustrative example. In some examples, control circuit 206 may set thetarget high A4 sensing threshold amplitude based on the A3 amplitudemetric 954, e.g., to a percentage larger than the A3 amplitude metric954 or by adding an offset to the A3 amplitude metric 954 and/or addingthe low A4 sensing threshold amplitude to the value of the A3 amplitudemetric 954. The added offset may be 0.2, 0.3, 0.5, 0.8, or 1.0 m/s² asexamples. During an atrial tracking ventricular pacing mode, all or avast majority of the motion sensor signal peak amplitudes represented inthe normal A4 event histogram 952 should not be detected as A4 events byatrial event detector circuit 240. Thus, control circuit 206 may beconfigured to set the high A4 sensing threshold amplitude to a valuethat is equal to or higher than the A3 amplitude metric 954 to avoidoversensing A3 events.

The second, bottom histogram 962 represents all A3 event amplitudesdetermined over the predetermined time period for accumulating histogramdata. Histogram 962 may therefore include A3 event amplitudes determinedfrom ventricular cycles with A4 events detected during the A3 window, A4events detected early after the A3 window ending time and “normal” A4events detected a threshold time interval after the A3 window endingtime (and possibly ventricular cycles with no sensed A4 event). Asobserved by histograms 952 and 962, higher frequencies of higher A3event amplitudes, greater than the A3 amplitude metric 954, may occurwhen all A3 event amplitudes are stored in the histogram 962, presumablydue to the effect of early A4 events and A4 events sensed during the A3window contributing to the A3 event amplitudes. Thus, the higher A3event amplitudes included in histogram 962, greater than the A3amplitude metric 954, may represent ventricular cycles having fusedA3/A4 events occurring during the A3 window or near the A3 window endingtime.

Control circuit 206 may be configured to determine the target high A4sensing threshold amplitude 964 at block 912 of FIG. 16 based on the A3event amplitudes stored in histogram 962 that are greater than the A3amplitude metric 954. These A3 event amplitudes that are higher than theA3 amplitude metric are shown over the range 966. In the illustrativeexample shown, the target high A4 sensing threshold amplitude 964 is setto a percentile of the A3 event amplitudes over the range 966. Forinstance, the target high A4 sensing threshold amplitude 964 may be setto the 10th, 20th, 25th, 30th or 35th percentile of the A3 eventamplitudes greater than A3 amplitude metric 954, shown by range 966. Insome cases an offset and/or the low A4 sensing threshold amplitude maybe added to the value of the predetermined percentile 964 to determinethe target high A4 sensing threshold amplitude. In other examples, thetarget high A4 sensing threshold amplitude may be set based on apercentile of all amplitudes stored in histogram 962. For instance,control circuit 206 may set the target high A4 sensing thresholdamplitude to a predetermined percentile, e.g., 60th, 65th, 70th, or 75thpercentile, of all A3 event amplitudes stored in histogram 962 and mayoptionally add an offset and/or the low A4 sensing threshold amplitudeto the value of the specified percentile amplitude. Accordingly, in someexamples, one of the histograms 952 or 962 may be populated fordetermining the target high A4 sensing threshold amplitude, but notnecessarily both. In other examples, both are populated as describedabove for use in determining an A3 amplitude metric from a first portionof the A3 event amplitudes, e.g., those identified from normal A4 eventventricular cycles, and a target high A4 sensing threshold amplitudebased on a second portion of the A3 event amplitudes, (e.g., thosegreater than the A3 amplitude metric).

FIG. 18 is a flow chart 1100 of a method for adjusting selected atrialevent sensing control parameters according to one example. Startingvalues of A4 sensing control parameters may be programmed by a clinicianor may be automatically selected by control circuit 206. For example,control circuit 206 may determine a starting value of one or more A4sensing control parameters at block 1101, such as the A3 window endingtime and the first, higher and second, lower A4 sensing thresholdamplitude values by determining A4 sensing control parameter metricsover an extended time interval or number of ventricular cycles whenpacemaker 14 is initially implanted. The starting values may be selectedby determining the A4 sensing control parameter metrics during anon-atrial tracking ventricular pacing mode that includes atrial sensingusing the motion sensor signal, e.g., a VDI mode as indicated at block1101. This pacing mode allows motion sensor signal features to beanalyzed for determining control parameter metrics over a relativelyextended period of time, e.g., one minute, two minutes, or five minutes,for selecting appropriate starting values of A4 sensing controlparameters for the patient. Techniques that may be used for selectingstarting control parameter values at block 1101 are generally disclosedin U.S. Patent Application No. 62/776,027, filed provisionally on Dec.6, 2018, and corresponding non-provisional U.S. patent application Ser.No. 16/703,047, filed Dec. 4, 2019, now published as U.S. PatentApplication Publication No. 2020/0179707 (Splett, et al.) and in U.S.Patent Application No. 62/776,034, filed provisionally on Dec. 6, 2018,and corresponding non-provisional U.S. patent application Ser. No.16/703,320 filed on Dec. 4, 2019, now published as U.S. PatentPublication No. 2020/0179708 (Splett, et al.) all of which areincorporated herein by reference in their entirety.

The starting values of the A4 sensing control parameters may be based onmotion sensor signal features that are determined over a relativelylong, extended time period compared to the N ventricular cycles usingfor adjusting the A4 sensing control parameters using the techniquesdisclosed herein. For example, motion sensor signal features used to setthe starting values may already be minutes or even hours old by the timethe starting values are selected. Furthermore, when the starting valuesare selected during the VDI pacing mode, A4 events will not beconsistently occurring during the A4 window. As such, after selectingstarting values at block 1101, before control circuit 206 switches fromthe VDI pacing mode to a permanent atrial-tracking ventricular pacingmode, e.g., VDD pacing mode, control circuit 206 may adjust the startingvalues of one or more A4 sensing control parameters over a relativelyshorter time interval, e.g., one to two minutes or over one or more setsof N ventricular cycles using the adjustment techniques disclosedherein. In this way, A4 sensing control parameters set to startingvalues based on “historical” data that may already be minutes or hoursold may be further optimized to an operational value that is based onmore recent, contemporaneously determined A4 sensing control parametermetrics. Adjusting the starting values to operational values just priorto switching to the permanent VDD pacing mode to deliver atrialsynchronized ventricular pacing promotes optimal A4 sensing controlparameter settings at the time of switching to the permanent VDD pacingmode.

After selecting a starting value for one or more A4 sensing controlparameters at block 1101, control circuit 206 advances to blocks 1102and 1104 to adjust the high A4 sensing threshold amplitude and the A3window ending time, respectively, from their starting values. It is tobe understood that blocks 1102 and 1104 may be parallel, simultaneousoperations. The determination of A4 sensing control parameter metricsrequired for determining the adjustments to both the high A4 sensingthreshold amplitude and the A3 window ending time may all be determinedover every N ventricular cycles and with subsequent adjustment of boththe high A4 sensing threshold amplitude and A3 window ending time at theend of each N ventricular cycles.

At blocks 1102 and 1104, control circuit 206 may continue to operate inthe non-atrial tracking ventricular pacing mode, e.g., in the VDI pacingmode, for a predetermined time interval, e.g., one minute, two minutes,three minutes or other selected relatively short time interval or numberof ventricular cycles, to adjust the high A4 sensing threshold amplitudeand the A3 window ending time from starting values determined over arelatively longer set up process to currently relevant values, which maybe referred to as “adjusted starting values.” At block 1102, controlcircuit 206 determines A4 sensing control parameter metrics and adjuststhe starting value of the high A4 sensing threshold amplitude usingtechniques described above in conjunction with FIGS. 12 and 15 . Forexample, the A3 amplitude metric may be determined as the median maximumamplitude that occurs during the A3 windows over N consecutiveventricular cycles. The high A4 sensing threshold amplitude value may beadjusted after every eight ventricular cycles based on the fourthhighest A3 event amplitude during the eight ventricular cycles. Duringthis process of adjusting the high A4 sensing threshold amplitude from astarting value during the VDI pacing mode, the high A4 sensing thresholdamplitude may be adjusted event N ventricular cycles without necessarilywaiting for an adjustment threshold to be reached as described inconjunction with FIG. 15 .

Control circuit 206 may determine a target value of the high A4 sensingthreshold amplitude based on the A3 amplitude metric. The starting valueof the high A4 sensing threshold amplitude determined during the set upprocess at block 1101 may be adjusted by a predetermined increment ordecrement toward the target value after every N ventricular cycles (oras controlled by an adjustment counter as described in conjunction withFIG. 15 ). This process of adjusting the high A4 sensing thresholdamplitude may be performed for an adjustment time interval, e.g., forone minute, two minutes, five minutes or other selected time interval,while operating in the VDI pacing mode. Control circuit 206 may storethe adjusted high A4 sensing threshold amplitude during the temporaryVDI pacing mode without applying the high A4 sensing threshold amplitudefor A4 event sensing during the VDI pacing mode. The adjusted startinghigh A4 sensing threshold amplitude value may not go into effect untilcontrol circuit 206 switches to an atrial tracking ventricular pacingmode such that A4 events are not being detected by atrial event detectorcircuit 240 until all A4 sensing control parameters are adjusted to anoperational value from the starting value.

At block 1104, control circuit 206 may determine the A3 event times,e.g., the latest low A3 threshold amplitude crossing by the motionsensor signal, which may be a negative going crossing, during the A3window (set to end at the starting A3 window ending time). The A3 eventtimes may be determined for N ventricular cycles. Control circuit 206may determine an A3 time metric from the A3 event times and a target A3window ending time as described above in conjunction with FIGS. 12 and13 . During the VDI pacing mode, the target A3 window ending time may bebased on only the A3 time metric and not an A4 time metric since areliable A4 time metric may be unknown during the non-atrial trackingventricular pacing mode. The starting value of the A3 window ending timeset a block 1101 may be adjusted based on the A3 time metric and targetA3 window ending time every N ventricular cycles at block 1104.

The A3 window ending time established during the set up procedure atblock 1101 may be based on the latest crossing of a test value of thelow A3 threshold amplitude. The test value may set to a predetermined,fixed or default value, e.g., 0.9 m/s². The low A3 threshold amplitudeused to determine the A3 event times at block 1104, however, may bebetter tailored to the patient when set by control circuit 206 to apercentage, e.g., 75%, of the starting value of the low A4 sensingthreshold amplitude value determined during the set up procedureperformed at block 1101, rather than a default value. Since the startinglow A4 sensing threshold amplitude determined during the set up processat block 1101 is better tailored to the patient, the low A3 thresholdamplitude set at block 1104 is also better tailored to the patient thana default value. The low A3 threshold amplitude set to a percentage ofthe starting low A4 sensing threshold amplitude may be a moreappropriate threshold for the given patient in determining A3 eventtimes and an A3 time metric for adjusting the A3 window ending time to avalue that is currently relevant at block 1104.

For example, if the starting value of low A4 sensing threshold amplitudeis set to 2.5 m/s² at the end of the set up process at block 1101, thelow A3 threshold amplitude set to 75% of the starting low A4 sensingthreshold amplitude is 1.9 m/s² instead of the default 0.9 m/s²threshold. This more relevant, patient-tailored, low A3 thresholdamplitude may be used during the A3 windows over N ventricular cycles atblock 1104 for detecting the latest low A3 threshold crossing times andadjusting the A3 window ending time. In this way, the A3 window endingtime is better optimized for the patient at block 1104 from its startingvalue. The A3 window ending time may be adjusted every 8^(th)ventricular cycle, or other selected number of ventricular cycles, for 2minutes (or other adjustment time interval) at block 1104 to arrive atan adjusted starting A3 window ending time that goes into effect as theoperational A3 window ending time upon switching to an atrial trackingventricular pacing mode (e.g., VDD pacing mode).

After adjusting the starting A3 window ending time and/or the startinghigh A4 sensing threshold amplitude to operational values during thetemporary VDI pacing mode at blocks 1102 and 1104, control circuit 206may switch to a temporary atrial tracking pacing mode (e.g., VDD pacingmode) at block 1106. The operational values of the high A4 sensingthreshold amplitude value and the A3 window ending value may be ineffect upon switching to the temporary VDD pacing mode.

The low A4 sensing threshold amplitude may be adjusted at block 1108from its starting value (selected during the set up process performed atblock 1101). During this temporary VDD pacing mode, the low A4 sensingthreshold amplitude may be adjusted from its starting value based on A4sensing control parameter metrics determined over every N ventricularcycles using techniques generally described above in conjunction withFIGS. 7-11 . Adjustments to the low A4 sensing threshold amplitudeduring the temporary VDD pacing mode with the contemporaneouslyoptimized A3 window ending time determined at block 1104 now in effectis expected to tune the low A4 sensing threshold amplitude to anoptimized value based on the A4 sensing control parameter metricsdetermined during the temporary VDD pacing mode. For example, the earlyA4 count, the applicable cycle count, the detected A4 count, and the A3window detection count may be determined by control circuit 206 at block1108 for use in adjusting the low A4 sensing threshold amplitude. TheseA4 sensing control parameter metrics are determined with the operationalA3 window ending time and operational high A4 threshold amplitudedetermined at blocks 1104 and 1102, respectively, in effect.

At block 1110, control circuit 206 may determine a rate smoothinginterval based on one or more ventricular cycle lengths occurring duringthe temporary VDD pacing mode. In some examples, a starting ratesmoothing interval is set to the programmed lower rate interval. Themedian ventricular cycle length over N ventricular cycles, e.g., eightventricular cycles may be determined by control circuit 206 at block1110. An adjusted rate smoothing interval may be set by control circuit206 to a predetermined interval longer than the median ventricular cyclelength, e.g., 100 to 150 ms longer than the median ventricular cyclelength. The rate smoothing interval may be updated every N ventricularcycles for a predetermined time interval, e.g., two minutes, during thetemporary VDD pacing mode at block 1110.

Control circuit 206 may enable rate smoothing during the temporary VDDmode to promote and maintain atrioventricular synchrony while A4 sensingcontrol parameter metrics are being determined for adjusting the low A4sensing threshold. The rate smoothing interval may be adjusted duringthe temporary VDD pacing mode at block 1110 at a higher frequency thanthe low A4 sensing threshold in some examples. For example, a median orfourth fastest ventricular cycle length may be determined out of themost recent eight ventricular cycles after every ventricular cycle on arolling basis. The rate smoothing interval may be set by control circuit206 to the updated median ventricular cycle length plus a specifiedoffset. Control circuit 206 may update the rate smoothing interval onevery ventricular cycle so that the VV pacing interval is set to theupdated rate smoothing interval on a beat-by-beat basis in someexamples. If the VV pacing interval that is set to the rate smoothinginterval expires, atrioventricular synchrony is expected to bemaintained with appropriate timing of the A4 window encompassing the A4event signal. By maintaining atrioventricular synchrony during thetemporary VDD pacing mode, the A4 sensing control parameter metricsdetermined at block 1108 for adjusting the low A4 sensing thresholdamplitude will enable control circuit 206 to adjust the low A4 sensingthreshold amplitude to an optimal value for the patient.

As such, it is to be understood that blocks 1108 and 1110 may beperformed in simultaneous parallel operations. For example, every Nventricular cycles control circuit 206 may determine the A4 sensingcontrol parameter metrics required for determining the adjustment to thelow A4 sensing threshold amplitude. Concurrently, on every ventricularcycle control circuit 206 may determine an updated ventricular cyclelength metric for adjusting the rate smoothing interval as often asevery ventricular cycle. The final adjusted values of the A4 sensingcontrol parameters determined at blocks 1102, 1104 and/or 1108 and thelatest updated rate smoothing interval may all go into effect asoperational values upon switching to the permanent VDD pacing mode atblock 1112.

After adjusting the starting value of the low A4 sensing thresholdamplitude and adjusting the rate smoothing interval during the temporaryVDD pacing mode over a predetermined time interval or number ofventricular cycles, control circuit 206 may switch to a permanent atrialtracking ventricular pacing mode at block 1112, e.g., the VDD pacingmode, with the operational values of the A4 sensing control parametersand adjusted rate smoothing interval in effect. In this way, startingvalues of the A4 sensing control parameters, which may be programmed bya user or determined during a set up process at block 1101 over arelatively longer time period, can be adjusted to operational valuesthat are more currently relevant values based on more recent motionsensor signal amplitude and timing features used to determine A4 sensingcontrol parameter metrics. The rate smoothing interval is set to acurrently relevant value based on the current ventricular rate. Theseoperational values of the A4 sensing control parameter metrics and therate smoothing interval go into effect upon switching to the VDD pacingmode to provide optimized atrial tracking ventricular pacing.

It should be understood that, depending on the example, certain acts orevents of any of the methods described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of themethod). Moreover, in certain examples, acts or events may be performedconcurrently, e.g., through multi-threaded processing, interruptprocessing, or multiple processors, rather than sequentially. Inaddition, while certain aspects of this disclosure are described asbeing performed by a single circuit or unit for purposes of clarity, itshould be understood that the techniques of this disclosure may beperformed by a combination of units or circuits associated with, forexample, a medical device.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored as one or more instructions orcode on a computer-readable medium and executed by a hardware-basedprocessing unit. Computer-readable media may include computer-readablestorage media, which corresponds to a tangible medium such as datastorage media (e.g., RAM, ROM, EEPROM, flash memory, or any other mediumthat can be used to store desired program code in the form ofinstructions or data structures and that can be accessed by a computer).

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPLAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

Thus, a medical device has been presented in the foregoing descriptionwith reference to specific examples. It is to be understood that variousaspects disclosed herein may be combined in different combinations thanthe specific combinations presented in the accompanying drawings. It isappreciated that various modifications to the referenced examples may bemade without departing from the scope of the disclosure and thefollowing claims.

What is claimed is:
 1. A medical device comprising: a pulse generator configured to produce electrical stimulation pulses; a motion sensor configured to produce a motion signal; and a control circuit configured to: control the pulse generator in producing the electrical stimulation pulses for delivering a therapy; set a plurality of sensing control parameters comprising a ventricular diastolic event window ending time; sense first atrial events from the motion signal during a plurality of ventricular cycles according to the sensing control parameters; determine a features of the motion signal for each ventricular cycle of at least a portion of the plurality of ventricular cycles; determine at least one metric of the motion signal based on the determined features, wherein determining the at least one metric comprises: identifying each ventricular cycle of the plurality of ventricular cycles that is longer than the ventricular diastolic event window ending time; and determining a count of the plurality of ventricular cycles that are identified as being longer than the ventricular diastolic event window ending time; adjust a sensing control parameter of the plurality of sensing control parameters based on the at least one metric; sense a second atrial event from the motion signal according to the adjusted sensing control parameter; and generate an atrial sensed event signal in response to sensing the second atrial event.
 2. The medical device of claim 1, wherein the control circuit is further configured to: set the plurality of control parameters by: setting a high sensing threshold amplitude extending to an expiration of the ventricular diastolic event window ending time; and setting a low sensing threshold amplitude starting from the expiration of the ventricular diastolic event window ending time after the high sensing threshold amplitude; sense each of the first atrial events during the plurality of ventricular cycles in response to an earliest crossing of one of the high sensing threshold amplitude and the low sensing threshold amplitude by the motion signal; and adjust the sensing control parameter by adjusting at least one of the ventricular diastolic event window ending time, the high sensing threshold amplitude or the low sensing threshold amplitude.
 3. The medical device of claim 2, wherein the control circuit is further configured to: determine the features by determining a time of each of the first atrial events sensed during the plurality of ventricular cycles; determine the at least one metric based on the determined time of each of the first atrial events sensed during the plurality of ventricular cycles by determining at least one of: an early atrial event count by counting each of the first atrial events that are sensed after the ventricular diastolic event window ending time and before a threshold time interval; a normal atrial event count by counting each of the first atrial events that are sensed at or later than the early event threshold time interval; and a ventricular diastolic window atrial event count by counting each of the first atrial events that are sensed before the ventricular diastolic event window ending time.
 4. The medical device of claim 3, wherein the control circuit is configured to: determine that ventricular diastolic event oversensing criteria are met based on at least the early atrial event count; and adjust the sensing control parameter based on the early atrial event count by increasing the low sensing threshold amplitude in response to the ventricular diastolic event oversensing criteria being met.
 5. The medical device of claim 3, wherein the control circuit is configured to: determine that regular atrial event sensing criteria are met based on at least the normal atrial event count; and adjust the sensing control parameter based on the normal atrial event count by increasing the low sensing threshold amplitude in response to the regular atrial event sensing criteria being met.
 6. The medical device of claim 3, wherein the control circuit is configured to: determine that atrial event undersensing criteria are met based on at least the normal atrial event count; and adjust the sensing control parameter based on the normal atrial event count by decreasing the low sensing threshold amplitude in response to the atrial event undersensing criteria being met.
 7. The medical device of claim 2, wherein the control circuit is further configured to: determine the features by determining a maximum amplitude of the motion signal after the ventricular diastolic event window ending time for each of the plurality of ventricular cycles that is longer than the ventricular diastolic event window ending time; and determine the at least one metric by determining a minimum one of the maximum amplitudes; and adjust the sensing control parameter by adjusting the low sensing threshold amplitude based on the minimum one of the maximum amplitudes.
 8. The medical device of claim 2, wherein the control circuit is further configured to: determine the features by determining a latest crossing time of a ventricular diastolic event threshold amplitude occurring before the ventricular diastolic event window ending time; determine the at least one metric by determining a ventricular diastolic event time based on the determined latest crossing times; and adjust the sensing control parameter by adjusting the ventricular diastolic event window ending time based on the ventricular diastolic event time.
 9. The medical device of claim 8, wherein the control circuit is configured to set the ventricular diastolic event threshold amplitude based on the low sensing threshold amplitude.
 10. The medical device of claim 8, wherein the control circuit is further configured to: determine the features by determining an earliest crossing time of the low sensing threshold amplitude; determine the at least one metric by determining an atrial event time based on the determined earliest crossing times; and adjust the ventricular diastolic event window ending time based on the ventricular diastolic event time and the atrial event time.
 11. The medical device of claim 2, wherein the control circuit is further configured to: determine the features by determining a maximum amplitude of the motion signal that precedes the ventricular diastolic event window ending time; determine the at least one metric by determining a ventricular diastolic event amplitude based on the determined maximum amplitudes; and adjust the sensing control parameter by adjusting the high sensing threshold amplitude based on the ventricular diastolic event amplitude and the low sensing threshold amplitude.
 12. The medical device of claim 11, wherein the control circuit is configured to: determine a target high threshold amplitude based on a combination of the ventricular diastolic event amplitude and the low sensing threshold amplitude; determine a difference between the target high threshold amplitude and the high sensing threshold amplitude; adjust a counter value based on the difference; determine that the counter value is equal to one of a maximum threshold and a minimum threshold; and adjust the high sensing threshold amplitude toward the target high threshold amplitude in response to the counter being equal to one of the maximum threshold or the minimum threshold.
 13. The medical device of claim 2, wherein the control circuit is further configured to: determine the features by determining a first plurality of maximum amplitudes of the motion signal preceding the ventricular diastolic event window ending time for each of the plurality of ventricular cycles having one of the first atrial events sensed after the ventricular diastolic event window ending time; determine the at least one metric by determining a first percentile of the first plurality of maximum amplitudes; determine a second plurality of maximum amplitudes of the motion signal preceding the ventricular diastolic event window ending time for each of the plurality of ventricular cycles; determine a target high sensing threshold by determining a second percentile of the second plurality of maximum amplitudes that are greater than the metric; and adjust the sensing control parameter by adjusting the high sensing threshold amplitude based on the target high sensing threshold.
 14. The medical device of claim 2, wherein the control circuit is configured to: determine that the atrial sensed event signal is generated before the ventricular diastolic event window ending time expires; and extend the ventricular diastolic event window ending time until a subsequent ventricular electrical event in response to determining that the atrial sensed event signal is generated before the ventricular diastolic event window ending time expires.
 15. The medical device of claim 1, further comprising a pulse generator configured to generate a pacing pulse in response to the atrial sensed event signal.
 16. The medical device of claim 15, further comprising: a housing enclosing the motion sensor, the control circuit and the pulse generator; and a pair of electrodes on the housing and coupled to the pulse generator.
 17. A method, comprising: producing electrical stimulation pulses for delivering a therapy; producing a motion signal by a motion sensor; setting a plurality of sensing control parameters comprising a ventricular diastolic event window ending time; sensing first atrial events from the motion signal during a plurality of ventricular cycles according to the sensing control parameters; determining a features of the motion signal for each ventricular cycle of at least a portion of the plurality of ventricular cycles; determining at least one metric of the motion signal based on the determined features wherein determining the at least one metric comprises: identifying each ventricular cycle of the plurality of ventricular cycles that is longer than the ventricular diastolic event window ending time; determining a count of the plurality of ventricular cycles that are identified as being longer than the ventricular diastolic event window ending time; adjusting a sensing control parameter of the plurality of sensing control parameters based on the at least one metric; sensing a second atrial event from the motion signal according to the adjusted sensing control parameter; and generating an atrial sensed event signal in response to sensing the second atrial event.
 18. The method of claim 17, wherein: setting the plurality of control parameters further comprises: setting a high sensing threshold amplitude extending to an expiration of the ventricular diastolic event window ending time; and setting a low sensing threshold amplitude starting from the expiration of the ventricular diastolic event window ending time after the high sensing threshold amplitude; sensing each of the first atrial events in response to an earliest crossing by the motion signal of one of the high sensing threshold amplitude and the low sensing threshold amplitude during a respective one of the plurality of ventricular cycles; and adjusting the sensing control parameter comprises adjusting at least one of the ventricular diastolic event window ending time, the high sensing threshold amplitude or the low sensing threshold amplitude.
 19. The method of claim 18, further comprising: determining the features by determining a time of each of the first atrial events sensed during the plurality of ventricular cycles; and determining the at least one metric based on the determined time of each of the first atrial events sensed during the plurality of ventricular cycles by determining at least one of: an early atrial event count by counting each of the first atrial events that are sensed after the ventricular diastolic event window ending time and before a threshold time interval; a normal atrial event count by counting each of the first atrial events that are sensed at or later than the early event threshold time interval; and a ventricular diastolic window atrial event count by counting each of the first atrial events that are sensed before the ventricular diastolic event window ending time.
 20. The method of claim 19, comprising: determining that ventricular diastolic event oversensing criteria are met based on at least the early atrial event count; and adjusting the sensing control parameter based on the early atrial event count by increasing the low sensing threshold amplitude in response to the ventricular diastolic event oversensing criteria being met.
 21. The method of claim 19, comprising: determining that regular atrial event sensing criteria are met based on at least the normal atrial event count; and adjusting the sensing control parameter based on the normal atrial event count by increasing the low sensing threshold amplitude in response to the regular atrial event sensing criteria being met.
 22. The method of claim 19, comprising: determining that atrial event undersensing criteria are met based on at least the normal atrial event count; and adjusting the sensing control parameter based on the normal atrial event count by decreasing the low sensing threshold amplitude in response to the atrial event undersensing criteria being met.
 23. The method of claim 18, further comprising: determining the features by determining a maximum amplitude of the motion signal after the ventricular diastolic event window ending time for each of the plurality of ventricular cycles that is longer than the ventricular diastolic event window ending time; determining the at least one metric by determining a minimum one of the maximum amplitudes; and adjusting the sensing control parameter by adjusting the low sensing threshold amplitude based on the minimum one of the maximum amplitudes.
 24. The method of claim 18, further comprising: determining the features by determining a latest crossing time of a ventricular diastolic event threshold amplitude occurring before the ventricular diastolic event window ending time; determining the at least one metric by determining a ventricular diastolic event time based on the determined latest crossing times; and adjusting the sensing control parameter by adjusting the ventricular diastolic event window ending time based on the ventricular diastolic event time.
 25. The method of claim 24, comprising setting the ventricular diastolic event threshold amplitude based on the low sensing threshold amplitude.
 26. The method of claim 24, further comprising: determining the features by determining an earliest crossing time of the low sensing threshold amplitude; determining the at least one metric by determining an atrial event time based on the determined earliest crossing times; and adjusting the ventricular diastolic event window ending time based on the ventricular diastolic event time and the atrial event time.
 27. The method of claim 17, further comprising: determining the features by determining a maximum amplitude of the motion signal that precedes the ventricular diastolic event window ending time; determining the at least one metric by determining a ventricular diastolic event amplitude based on the determined maximum amplitudes; and adjusting the sensing control parameter by adjusting the high sensing threshold amplitude based on the ventricular diastolic event amplitude and the low sensing threshold amplitude.
 28. The method of claim 27, comprising: determining a target high threshold amplitude based on a combination of the ventricular diastolic event amplitude and the low sensing threshold amplitude; determining a difference between the target high threshold amplitude and the high sensing threshold amplitude; adjusting a counter value based on the difference; determining that the counter value is equal to one of a maximum threshold and a minimum threshold; and adjusting the high sensing threshold amplitude toward the target high threshold amplitude in response to the counter value being equal to one of the maximum threshold or the minimum threshold.
 29. The method of claim 18, further comprising: determining the features by determining a first plurality of maximum amplitudes of the motion signal preceding the ventricular diastolic event window ending time for each of the plurality of ventricular cycles having one of the first atrial events sensed after the ventricular diastolic event window ending time; determining the at least one metric by determining a first percentile of the first plurality of maximum amplitudes; determining a second plurality of maximum amplitudes of the motion signal preceding the ventricular diastolic event window ending time for each of the plurality of ventricular cycles; determining a target high sensing threshold by determining a second percentile of the second plurality of maximum amplitudes that are greater than the metric; and adjusting the sensing control parameter by adjusting the high sensing threshold amplitude based on the target high sensing threshold.
 30. The method of claim 17, comprising: determining that the atrial sensed event signal is generated before the ventricular diastolic event window ending time expires; and extending the ventricular diastolic event window ending time until a subsequent ventricular electrical event in response to determining that the atrial sensed event signal is generated before the ventricular diastolic event window ending time expires.
 31. The method of claim 17, comprising generating a pacing pulse in response to the atrial sensed event signal.
 32. A non-transitory, computer-readable storage medium comprising a set of instructions which, when executed by a control circuit of a medical device, cause the medical device to: produce electrical stimulation pulses for delivery a therapy; produce a motion signal by a motion sensor; set a plurality of sensing control parameters comprising a ventricular diastolic event window ending time; sense first atrial events from the motion signal during a plurality of ventricular cycles according to the sensing control parameters; determine a features of the motion signal for each ventricular cycle of at least a portion of the plurality of ventricular cycles; determine a at least one metric of the motion signal based on the determined features, wherein determining the at least one metric comprises; identifying each ventricular cycle of the plurality of ventricular cycles that is longer than the ventricular diastolic event window ending time; determining a count of the plurality of ventricular cycles that are identified as being longer than the ventricular diastolic event window ending time; adjust a sensing control parameter of the plurality of sensing control parameters based on the at least one metric; sense a second atrial event from the motion signal according to the adjusted sensing control parameter; and generate an atrial sensed event signal in response to sensing the second atrial event. 